People split for all sorts of reasons but likewise peoe stay together for all sorts of reasons. I have trouble with the term 'better' because its such an arbitrary term. Family dynamics has nothing to do with whether or not the children came from the same genetics.
ETA I hate it when people assume others have made 'bad choices' simply because a relationship ended... It's very insulting. There's lots of stressers in life that can have a huge impact on individuals. It's life, things change. And also in many situations people show real courage and a true sense of self to walk away from relationships - it doesn't mean children involved will suffer at all. Children can suffer under any type of family situation take it from me!