My wife and I dated through undergrad, broke up at "get married or break up." And ended coming back together as adults with more life and relationship experience. Based on plans, we never should have been back in the same city. And when we started hanging out again, neither expected a relationship, just a friendship still being sick of each others' sh. But we had both grown up. We're 24 years into our relationship now. I think it's going well.
That's all purely anecdotal, though.
I don't think real math could be done with this. Success isn't measurable. People often split due to $ or tragedy. People stick it out, miserable the entire time. It walks into Aristotle's eudaimonia. You can't know until it's over. Even then, quantifying quality is folly.