Ugh, really? But Athos/Milady was a toxic relationship. I was so happy to see Athos find new joy in life and love in S3. The way he grew and matured in S3, because of the closure he'd found for his relationship with Milady in S2, was just lovely. Milady's return at the end of the season was completely unnecessary, narratively. I was perfectly content to think of her leading a life of leisure in England. At least she didn't really disrupt Athos' relationship with Sylvie, though.
Exactly! The way it plays out in season 3 makes perfect sense for the characters and their relationship and history - they have a huge amount of baggage which can only ever hurt them if they try to remain together, the only way for either one to truly be happy was to let go of the past and make a fresh start. Which Athos has done, and it is lovely development for him, and I like to think that perhaps in the S4 that will never be made, Milady too would put the past behind her and actually move past it, instead of wallowing in it.
But going into the Musketeers tag it is clear that a very vocal segment of the fandom don’t care that the relationship was toxic, they loved the chemistry and passion regardless and were convinced that it should be endgame. Unhealthy relationships are very popular in fandom. And they are furious that Sylvie took Athos away from Milady, as they see it, instead of leaving him to redeem her.
Which…the whole point of a redemption story is that the individual has to want it for themselves, which Milady never really has. And the brief history of her time in England proves that point. If Athos took her back she might try to be a better person, because it’s what he’d want, but he’d never be able to trust her - whereas Sylvie actually is a good person, and exactly what he needed to put the tragedies of his past behind him and move forward with his life. (He will never have to worry about Sylvie casually murdering someone who annoys her, for starters). He couldn’t be Milady’s crutch, it would have destroyed him. He deserves the brighter future he will have with Sylvie. And Milady too will be better off in the long run now that the tie between them has been finally broken.