That is a well balanced critique of accelerators.

One possible defense of incubators could be that their value increases with time.

The first class of companies may not benefit much, but as time goes on, some of those companies will certainly succeed. And those those successful companies will offer value to the current/future batches.

Of course, YC has the advantage of already having several successful alumni. And like the Matthew Principle says, those who have will be given more.