If they had stayed together romantically, it would have fallen apart—I think he was sincere when he confessed that he loved her at the end of season two, and perhaps he had sincerely loved her for a long time (a dude with no feelings for the girl he secretly boned doesn’t sucker-punch his colleague in the face for making fun of her weight), but we’ve seen what he’s like with the women he has relationships with. There’s a pattern, where he goes along happily for a while, boyishly sweet and charming, and then he has a bad day and has to tear someone else down, and usually that’s someone who won’t fight back. He broke Peggy’s heart long before she broke his.
Peggy started out insecure, but she has grown a lot in self-confidence and recognition of her own worth over the course of the show, and you know she’ll be okay in the long run, no matter what she tries to do. She’s resilient. Pete Campbell isn’t so much. Peggy’s gone through a lot of misery and healing and it’s made her harder to break, but even though Pete’s grown a lot, he’s still immature, and difficulties seem to make him more fragile and desperate.
I think they could be great friends, though. Not that Peggy’s in particular need of a guy like Pete, but they understand each other, and Pete could benefit from a mutual, genuine friendship with someone who won’t sleep with him, but would be honest with him in a way that he could handle, call him out when he needs it (which, let’s face it, is a lot), and care about him anyway. Sort of like Don and Anna. Accept the broken aspects of each other and love each other anyway, unto the bitter end. I guess that is my official Mad Men ship. Bittersweet friendship shipping.
Sort of an aside, but I find it fascinating that a running theme of this show is the married men seeking out other women, usually mistresses or prostitutes, and treating them like psychiatrists. With the wife you can’t rock the boat too much, she’ll judge you and she might even leave you, and your male friends are people you have to impress, but some woman who has no real stake will listen and accept you even when you tell her flat-out what’s wrong with you.