My parents came close to divorce in their 50s (financial infidelity and some neglect) but stuck it out because they were super religious and my sister was very young, then things got really good between them again and now that my father has hit 80 they fight quit a bit..and my mother mentions divorce more often. They are spending too much time together for sure, my dad doesn't have enough to keep himself busy and is driving my mother nuts...nothing like having someone come into your garden and start throwing out "weeds" that were your entire crop of sweet potatoes. They also fight over money (ie. my father is of the adage I MADE the money so I control the money) and my mother put up with it for decades.. however as they age, that isn't flying any longer as she realizes she may outlive him by 20 years and he talks about giving it away upon his death to charities without regard to her very real need of it.
So divorce is definetly on the table and I waffle if it would be a good thing given my mother really does need to protect herself financially. Given she is 10 years younger than my father, her "1/2" of his SS is substantially smaller as I never knew it didn't account for inflation. So yeh, up to now there was always a kid at home (many returned multiple times) but now that they have all left, there is a lot of "perfect family" moments from the outside but internally, I know my mother may reach her limit one day and talk to an attorney. It happens..
I assume this is true with many grey divorces, you put up with stuff and life gets in the way and you just let it go.. as you get older, empty nest, those things seem bigger issues and your around each other way more time without any buffer and you either deal with them or you snap.. enough is enough.