This is probably not true, though. Many of the people at YC have said, many times, that what they're trying to do is find the next Dropbox, and everything else

This is probably not true, though. Many of the people at YC have said, many times, that what they're trying to do is find the next Dropbox, and everything else doesn't matter. They pretty explicitly talk about how hard to do this is, but how their efforts should go towards that anyway. The thing is, it's hard to to tell, especially from the outside, whether a company can viably become a billion dollar company. Sure, you've seen plenty of YC companies that ended up either not going for it, or not looking like it's possible for them to end up as unicorns, but this is in some cases too late - you aren't hearing their pitch, in which they explain why they could be the next billion dollar startup. Investors who want to make money take seriously the idea that one real success is the only thing that matters.