Do they though? It seems every demo day some VC posts up a blog about how they are not fans of demo day anymore for X, Y and Z reasons and that they're no longer attending.
Don't get me wrong YC is great and all but you make it sound like most of the VCs need YC but so many companies outside of YC get funding I'm not convinced that's even close to the truth.
Dealflow is everything in the VC community, but this is pure hyperbole. Statistically speaking, there are way more companies that drive VC returns that come from outside of YC than the ones that come from it.
This seems to be the case so far (YC has kept track of "problem VCs" for a long time), but I don't see why it should be. One of the few things more powerful than "if you sexually harass a YC founder, you'll be shunned by all future YC founders" is "if you sexually harass a YC founder, you'll be shunned by everybody".