What you allude to in your first statement is incorrect for most cases, IMO. Staying together doesn't necessarily mean letting it decrease your happiness. It might just mean you have to get your happiness from a different place.
Often in life you're not presented with win-lose situations. Instead, you're presented with lose-lose situations, and it's your job to choose the least worst solution for everyone involved.
I used to feel this way, but as I've gotten older, I've come to realize that there are a large number of things in life that are completely outside of my control. Accepting that doesn't mean "settling", it means that I've realized that I can't control those situations, but I can control my reaction and how it affects me. Marriage may or may not fall into that category for some people, but generally the behavior of others, health issues, macroeconomic events, etc. are things we can't control and must learn to live with.
I don't know why this was downvoted. I don't agree with the post completely but I don't find it snarky or anything.
To the post itself: It's a bit more complex than that. If the kids mean a lot to you then splitting up means seeing them less, making the "trade" times stressful for everyone, etc. It's a trade off like many things in life.
But I do agree that there are times that staying can be more damaging than leaving.