What I find difficult about the phrase 'staying together for the kids' is that it has undertones of that situation being somehow good (for the kids) and selfless. And by contrast, that splitting up is somehow bad (for the kids) and selfish.
Most people making these decisions are trying to consider everything that plays into the situation - finances, stability, personal emotional health, emotional health of the children.
I think 'staying together for the kids' is a lazy opt out way of describing it (to yourselves or others). There are so many differnet ways of working out - for better or worse - (maybe even for richer or poorer). Just in a few moments of thinking, I've come up with the examples below:
I have a friend who is staying (3 teenage kids) because if they split each of them (Mum and Dad) would have a one bedroom flat and that would be totally impractal. But she is saving and has her plan for when your youngest leaves... She is clear though that she is staying for the financial practicality of the whole family.
I left when dc1 was 9. They now tell me there is not enough researched and documented about the positives of healthily split families.
My best friend and her dh 'stayed together for the kids'. Both had affairs. In the short term it was awful. 10 years later they are the most loving and happy and contented couple, and they and their dc are a tight family unit.
My friend's parent's stayed together for the family in a toxic relationship. It really messed up their kids' perceptions of love and family and realtionship. After the dc left home the parents continued in mutual disharmony. At the age of 80, friends dm suddenly left her husband. "I've only got a few years to live, I don't want it to be like this, I want my last years to be happy." That had fucked the dc up a bit more.