It appears to me that almost all companies theoretically have a chance of growing that big. You never know what area they'll pivot into and if they'll find unexpected growth along the way. And so I think that there wouldn't be YCombinator's top 10% without the long tail of lesser-valued (and even longer tail of failed) startups. A lot of it comes down to the people who run them and pure chance. So it doesn't make much sense to say that a startup will not be worth $SOME_VALUATION at some indeterminate point in the future. We see these 180° changes a lot when looking at successful companies in retrospect, and they can deliver order of magnitude differences in market cap.
Of course, if you're not building a startup but an "ordinary", self-sustainable company (37Signals is a widely known example), then VCs and YCombinator are not for you. But that kind of follows from the definition.