People's feelings change, but how much integrity somebody has (whether they keep their word) is a part of their character, and people's character changes less frequently.
Especially when there’s even a financial incentive to getting out of a relationship you no longer enjoy. Everybody has a price, and seeing your marriage failing and a financial bonus to getting out of it changes opinions and convictions pretty fast.
This is especially true in countries that openly favour one of the spouses. For exaple ones where the legal framework doesn’t consider the contribution each spose had to the marriage, and one of them will invariably receive financial aid.
Integrity doesn't change, it's either there or it isn't. If you start a business with somebody, then after 10 years they suddenly backstab you and take your business, we don't say "they changed", we say they had no integrity to begin with. Integrity that somebody won't stick to is almost by definition not integrity, as "keeping one's word" implies keeping it unconditionally.
My first marriage ended in divorce, and we settled entirely amicably, out of court, because we both thought it would be pretty horrible to drag the other through the court system just because it turned out we wanted different things in life.
To assess integrity, it's not enough to look at how someone treats you, the person they love. It's also important to look at how they treat things they don't care about, how they treat things they hate, how they approach things they don't want to do but have to do anyway, how they treat people under their power like employees or serving staff. How they treat the promises they don't have to keep. Because if the love dies, that may be how they'll treat you.
Yes, and everyone thinks they're a great judge of character and integrity and they won't get backstabbed. Just the unenlightened and unwashed masses around them who don't know how to really look at a person and determine how trustworthy they are.
Then life goes on and someone who they thought was decent goes through a difficult time in life and backstabs them.
Even ignoring the very simple fact that people do change, there's the undeniable fact that the circumstances around a person change and it can and does push people to do things that they typically wouldn't do.
People who've lived comfortable lives and never had money troubles think they'll always be fine, but the minute their finances take a slight dip, they panic. It's a problem they never expected and it's probable they'll make some irrational decisions. Or maybe that person you've trusted all your life found out their brother has cancer and they're desperate to do anything to pay for the bills.
Shit happens. People try to adapt to it however they can. Thinking you'll avoid problems just by judging their integrity with a firm handshake and getting to know them is setting yourself up to be screwed over by people who know how to find an easy target, if not setting yourself up to be one of those virtuous people who screws over other people because your motives are good and theirs aren't.
Integrity, like any human characteristic, is a distribution; some people have more of it than others. If you just view everybody as the same, you're more likely to end up with somebody with average integrity, as opposed to if you actively searched for people displaying greater integrity. Similarly, assessing people is a skill that varies greatly among people, and if you just take the view that everybody is the same at it, you'll have no motivation to develop that skill yourself.