It's an idea that betrays a profoundly defective concept of marriage - that a lifelong relationship should seem easy and natural and that if it's not so perfect that you're absolutely certain that it's the best possible relationship you could ever have, the solution is to end it and start from scratch with someone else. It's a rejection of the idea that living with anyone is inherently difficult and requires a great deal of hard work, that people get cold feet and seven year itches, that people take things for granted, that the grass always seems greener. If divorce were genuinely a good thing, we'd expect post baby-boom generations with high divorce rates to be much happier and more satisfied with their relationships than their parents and grandparents; Instead, the opposite is true.
Sorry, but this is just a delusion. The "rate of happiness" in relationships was simply not measured until the sexual revolution of the '60s/'70s: because of asymmetry in roles, married women were just not allowed to have an opinion on the matter.
Also, there was very little acceptance for the possibility of unhappiness: if you were correctly following all social and religious mores, "of course" you were happy. Being unhappy was a weakness which could not be displayed nor tolerated.
Things change for a reason. Idyllic Arcadian scenarios are always, invariably, delusions concocted by incomplete information.
You mean this had nothing to do with the quality of their relationship? Otherwise, I agree with you.
I seem to recall a study that showed many partners were much happier with each other after the pressures of marriage were removed from their relationship. I think there is a strong argument to be made that the actions and later regret of those people you refer to is a direct result of having no "escape hatch" in marriage. Destroying everything you have is often the only way out, even if it would have been prudent for the couple to just end the marriage and keep the relationship. But that brings us back to the parent's point: Who ends a marriage to increase the happiness of the relationship? Nobody. Divorce implies an end.
Only if nothing except divorce had changed. For example, it could be that people nowadays have greater expectations for their marriages.