I'm reminded of a scene from Scrubs where Turk tells JD: "I love how kids of divorced parents swear they have the market cornered on family dysfunction...". Point being - I don't view "not divorced" as a significant metric for a successful / happy marriage.
Louis CK has a good bit about this too: "Divorce is always good news. I know that sounds weird, but it's true because no good marriage has ever ended in divorce. That would be sad. If two people were married and ... they just had a great thing and then they got divorced, that would be really sad. But that has happened zero times."
The quality of marriage is neither binary nor one-dimensional, it varies over time, and most importantly, can be influenced for the better or worse.
Its graph over time is also also not monotonic decreasing.
I fear that some marriages get divorced at a local minimum.