Hmm, interesting thread. I'm early 50s and no plans to leave DH. Marriage not perfect, but it's interesting that a lot of people who have left/are leaving, talk about being fed up of carrying the load. I've never felt like I'm carrying all the load, so maybe that's why I'm fairly happy. DH does all the cooking, and has always helped out with the kids a lot, from doing night feeds as babies to ferrying them everywhere as teens, and dealing with crises now as young adults. I suppose I carried more of the mental load of knowing what kids needed for school/childcare when they were younger, but that's gone way now. I also look after our finances (even though he earns more), but then I like that because it means I control the money (he has absolutely no interest in money, I could spend all our savings tomorrow and he wouldn't notice).
We definitely rub each other up the wrong way though eg. I like things clean and tidy, he likes clutter.
I think with age there is a feeling of "I've only got 20 years or whatever left, so I'm going to spend that time doing things I enjoy", which means there's less compromising. A simple example would be watching a film (but equally applies to bigger things like holidays/where to live etc) - whereas when younger we would have sat through watching a film that we didn't like for the sake of spending time with the other person, nowadays I don't want to waste that time, so I leave him to watch is old war films (or whatever) on his own. I suppose if one half of a couple is feeling like they're having to live a life where they're not happy most of the time, then they'll get fed up of compromising, which means they then have to split.
I don't think my life would change much if we split. I do what I want within the relationship anyway, as does he. I'd just be lonelier living on my own.