Would they really bother rejecting a lot of people off the waitlist? I would expect them just to do nothing rather than spending the time rejecting a lot of people. What if a few more people withdraw, and now they don't have any remaining waitlisters to admit? It seems like they would only reject someone off the waitlist if they realized later that they made a mistake by waitlisting them, and that they would never admit that person under any circumstances. But, what do I know? (answer: nothing)