Sure, long-term relationships can be a terrible experience. But I have my doubts whether your neighbors really still care about each other. (I know, no true Scotsman, but there really is "caring" and "staying together for the kids".) It's very easy to become unhappy in whatever way, but I think it's easier to become unhappy on your own, independent and unaffected by the concern of and for other people, than it is in a caring relationship.
And you are of course right - you can find unhappiness even when caring deeply about people. I just think that it's fair of me to say that if chances really are so equal, I'd rather live in the world where people try to care.
It's simply easier to turn a net quality of life increase for society from people who are into caring about each other than from people who don't. In fact, I think the second model, while popular for a while, has quite thoroughly collapsed in the recent years.