That is thoughtfully provocative. I have literally zero evidence to go from, but I suspect that Y Combinator is like Harvard or Yale: they could get rid of the group that they actually admit and go for the next tier, then achieve similar outcomes. (In other words: there are so many strong applicants that they can't take them all.)
This is just to say that even taking YC's cast-offs could be perfectly acceptable. YC is dominant for now, but it's not hard to imagine a world in which there are a few accelerators which are true YC peers (just like Harvard, Yale, Princeton, and Stanford are variously preferred by different people with real differences in preference).