The issue with the ending and its perception
It is a well-known fact that all what was in 5 season, at best popped out as the idea in 2007-2009, and most likely exactly in 2007-2009 there was only the idea of Jack getting a love-interest, returning to the past and losing the love-interest (although they wrote the outline of their movie or whatever in these 2007-2009). And there still were weird and vague details, which give a legitimate assumption that the animated movie was widely different from 5 season. Simply saying, nothing of what was in 5 season (and even the animated movie) was planned from the beginning, it haven't existed in the classic seasons and was never planned to be. 5 season doesn't give impression that it was planned for years and existed for many years, it looks like a rushed and bad fanfiction with Tartakovsky's self-insert Mary Sue and embodies all the most stupid and mediocre cliches from the mind of a person with bad taste and lack of talent, passion and imagination, as if it was created not even in 2012, but exactly in late-2015.
Better if 5 season was a stand-alone and self-contained independent reboot/remake with own separated canon and universe. Or as a separated show with different setting, universe, characters and with Ashi as a protagonist, because everything about her feels like from another cancelled show and shoehorned into Samurai Jack universe.
And although in interviews about 5 season Tartakovsky told that he had no ending in mind during working on the classic seasons and came to the idea several years later, certain classic episodes hinted the vague ending with pretty clear thematic shade — Jack will stay in the future and fight for this future and his friends, avenging his family and all tormented victims of the past. The episode with the Time Portal Guardian hinted that pretty well and clear, 'cause it looked more like a test for Jack, rather than irrefutable prophecy.
Plus, you could notice that in 4 season, specifically after the Aku Infection episode (and this episode really was felt like a character growth and arc), there were no episodes with Jack trying to find time portals or artifacts with time-travelling ability.
And, well, maybe it will sound cynical, but for Jack it makes zero sense to return and to save his parents and kingdom — Jack's parents and people of his kingdom are literally sick and exhausted old people with irretrievably undermined health, who will die a day earlier or later anyway, there is no point in saving them.
By this logic, Jack has to move to the friggin time of the "Birth of Evil", when the newborn Aku was innocent, weak and inexperienced, and nobody was harmed.
The idea with returning to the past and changing the past and hence the future is wrong and immoral in all senses, it makes Jack literally a murderer, who commits genocide of all habitants of the future, his friends, who considered him as their savior, and such ending erases the classic seasons, as if they never existed and were just a fever dream (like, as if Jack was knocked out by Aku for a second and saw all these adventures, but woke up and defeated him or whatever), devaluing the classic seasons and Jack's character growth and progress. Not to mention that after creating the time paradox Jack should disappear himself as a paradox.
And back in early-2015 Jim Zub and Andy Suriano have realized and understood that. They themselves considered the "Jack returns to the past and defeats Aku" ending very banal, happy, childish, immoral due to creating time paradoxes and erasing all Jack's growth, friends and adventures in the future. They wanted the mature ending with respect to the classic seasons. That's why they developed the idea from the episode with the Guardian, and that's why they created the bitter-sweet yet hopeful ending in the comics. The ending is open, Jack could defeat Aku, Jack could just imprison Aku either in the stone tree or even inside of himself (hence there's a chance for Jack to learn more about Aku's past and hence lead the demon to the path of light or whatever), or both Jack and Aku could die in the final battle — it doesn't matter, the core of this ending is in Jack's growth and his positive affect on this world, on this future and Jack's friends, how Jack himself decided to do this, he gave hope to them and power to not give up and fight for the happy life and this new world. I am not a fan of the comics, but I respect and enjoy its certain story moments, because they are really strong and fit to the character and his arc. And they actually keep the main duo in-character, without sucking out sudden love-interest or bastards or infected fanatics etc.
Not to mention the awesome arc with the lost sword, which was way better, epic, logical and emotionally powerful than in 5 season.
I find pretty ironic and amusing, how Tartakovsky always insists that his ending is the right, mature, bittersweet and tragic one and with the great sacrifice/loss, and how he tries to redeem and to excuse his ending in 5 season, and that in his vision everything else is very banal, happy and not-fitting to Jack and his story. Seems to me, he had a chip on his shoulder and was offended by the ending in the comics, and how their authors told everything what's wrong with that idea and actually respected the original seasons.
So, what's wrong with Tartakovsky's ending? It literally happens accidentally and popps out from nowhere, it happens NOT by Jack's will and decision, it is anticlimatic and stupid and treats audience like idiots, it gives no respect to Jack and especially to Aku, degradating them, it is purely immature, chilish and happy, because Jack's family, habitants of Jack's kingdom, Jack's teachers and classmates from the past (i.e. Jack's era) are all young and alive and healthy (even his parents), the kingdom suddenly is not destroyed, i.e. Jack got all what he kinda wanted. Sugar-coating happy ending! And on another side of a coin we have immoral and morbid crap — because Tartakovsky doesn't give a single f*ck about the mass genocide of the future with all its habitants (including Jack's friends). For him there's nothing wrong, because most part of habitants are not humans, I guess. In his paradigm it is good and okay, as well as okay to turn Jack into a fanatic, who never doubts and always thinks he's right in everything he does and has absolutist worldview (not to mention that if Jack is not immortal anymore, he and his adventures will be forgotten and erased from history, there's no global impact especially with the fact that humankind still will commit crimes no matter what; and if Jack is still immortal, he himself will become dictator — the most dangerous and terrible evil is the evil under the guise of virtue). Such ending could work only if the message was "Don't trust, always doubt, watch and check. You have to be careful. The one, who claims to be the savior from the forces of evil, can be this evil or even worse than the previous dictator." (especially on Ashi's example, since she idolized Jack and died for him, also killing the entire world), but there's nothing of it, no condemning, the author presents it as the saving of the world and that "Jack is the only one who has memories about them" (i.e. about the ones who never existed). So, where's the tragedy, sacrifise and loss?!
In Tartakovsky's eyes — the death of Ashi is what makes his ending so powerful, bittersweet, mature and tragic "We can't be entirely happy", so he sacrifices the logic for the sake of this. Simply saying, 5 season and its ending focus strictly on Ashi, making her Mary Sue, plot device, the main character, the true chosen one, the sudden bastard princess of Earth (although I still refuse to think that these seven b*tches are somehow related to Aku, they are just infected humans, nothing more; and 5 season itself debunks the idea of Aku being their father/creator, not to mention Tartakovsky himself stated the seven sisters are humans, so leave Aku alone), and she's the one, who throws into the conflict she knows nothing about and had nothing with it and resolves it. She is the center of 5 season and its ending and "drama" — for example, there's no scene of how Jack hugs the parents he finally met after 50+ years, he and all other characters mourn the f*cking Ashi. I remind, Ashi erased the future and genocided all its habitants (including Jack's friends from the future), and she perfectly knew what she is doing. Yep, Tartakovsky legitimately wants the audience to feel bad for the fanatical murderer, who killed thousands and erased the entire world in favor of the guy, who killed her sisters couple of days ago. And what's ironic, in 6 episode Ashi gave the hope to Jack by saying that he helped to lots of habitants of the future, and this aspect raises even more questions to why she decided to send Jack to the past and to allow to him to kill Aku and all Jack's friends (she even smiled during opening the time portal).
Here, here and here I covered in details what's wrong with Ashi, and also covered double standarts, how the tragedies of Jack and Aku were totally ignored, and how all their backstories, arcs and attention were given to Ashi. And that it is okay to doubt only specific selective dogmas and "higher forces", I guess. And "forcefully tragic or bittersweet" is also what ruins stories, because it devalues what happened previously and also treats audience like idiots, cheating emotions. As well as it is a total absence of respect to especially own story, when you treat it like "I want audience to either adore or hate it".
I can not for the life of me understand, why audience enjoyed it and why Tartakovsky sees nothing wrong with the finale and contradicts to both the classic seasons, to own words and to what happens in the friggin 5 season itself. It is a degradation.
So, the fact that Tartakovsky makes the new canon ending, where Ashi somehow doesn't die, and she lives with Jack as an empress happily without any punishment for her crime (honestly, now it doesn't surprise me at all, since Tartakovsky considers as poor innocents the friggin tribe of murderers, enslavers and conquers in interview about 2 season of Primal — because they're humans and have children, I guess).
I roasted the game here, here and here.
Honestly, looking at how they butchered Aku in 5 season by creating the sudden cult from nowhere and the seven sisters, it will be hilarious, if they come with the idea that even in the past (i.e. Jack's era) some cult still exists and some crazy female zealot prepares her "hybrid" sons/daughters against immortal Jack and Ashi in the style of Simba's Pride. 5 season already looked like a Simba's Pride clone, so I won't be surprised to that madness.