Put simply, what you say is nothing more than an ideal. You say " if both parents are working hard at a relationship..........." But the word you use is 'both' Do you understand that you can't control other people's actions? Sensible people have to work around reality.
You can disagree, but without superpowers, you cannot control somebody else's behaviour. That's the huge GAP in your theory. People you describe don't have to split up.
I am very reasonable and easy going and capable of compromise and of putting the children first and wanting what's best for them, and making sacrifices for them, but I am absolutely powerless to make my x operate under the same set of values and criteria . Things aren't so bad now that we've been apart for a long time. But it was the being together that made him incapable of separating getting one over on me, winning, showing me - from thinking first and foremost of the children's best interests.
Reading your posts I'm sure you mean well and want the best for children but your posts show that you do not understand that dynamic at all.
My children and millions like them would if I were still living in misery under the same roof as their father be busy trying to deflate tensions, tiptoe round land mines, stay quiet, avoid saying anything that could get me 'in trouble'.. As things stand, in two households, mind and their fathers, there is peace. NOt a lot of unnecessary communication between the two households but there is peace and because that old dynamic of him trying to win / beat me has died, now I think, ONLY NOW THAT we have no relationship at all, can he be on the same page as me putting the children first.
If you don't understand it, I envy you, but you ought to try, because what you are saying so dogmatically is just so inapplicable to the people who have relationships that aren't working.