"There's a difference between "no one wants to rot in a bad relationship" and "no one wants to put the effort and time to make a relationship work", though."
I am not sure, if I understand you right, but you seem to romantize the old version of marriage relationships.
Well, as far as I know, they had only very rarely something to do with love, but rather with violence and domination and "making the relationship work" meant beating the wife to submission.
And one of the written duties of a wife were sex, whether they wanted, or not, so basically rape.
And the thought of leaving an abusive partner was considered sin, with real punishment (sometimes torture to death) for making those thoughts real.
It has not been even so long ago, that these laws were abolished in the west. And to my knowledge, they still exist de facto (or even de jure) in many parts of the world.
The only reason I can think of, of "making a relationship work", is when there are already kids, who need stability. Then both adults should get their act together and work it out like responsible, mature people. But in the cases I have witnessed, the children usually were acting more mature, than their parents.