Here’s something else that got overlooked in the finale; Jack doesn’t age anymore. Sure he’s back in the past in his own time, but he lived 50 years in the future and his body stayed exactly the same. He’s in his 70s but looks like he’s in his 20s. That was a side effect of his first trip through time, and now he’s traveled through time twice.
What does this mean for Jack now? Is he still ageless? Did the second trip undo the effects of the first? Or did it do something else to him? Is he twice as ageless? Is he immortal? Is he going to live forever exactly the same while everyone around him grows old and dies?
God that would be an even more depressing ending to Jack’s saga; he saved the world but he’s still doomed to be alone in the end. And that’s bullshit.