Sure, there are still some older married couples who have been married 50+years, but they are few and far between. From experience, in order for a long long-term relationship to last, both people have to decide that the benefits of staying outweigh the benefits of leaving. Sometimes, staying is just simpler after sharing life events like children, in-laws and health crises too.
I do wonder if others in extremely long relationships have second thoughts or think about what ifs occasionally. They must, right?