From YC's perspective there are two ways in which they win when they invest in a company. On the one hand they make a return on their investment, classic straight-up utility maximizing economics. On the other hand they help a startup succeed, which is part of their overall purpose aside from raking in fat stacks of filthy lucre.

Even if a company succeeds without YC that doesn't mean that YC's potential involvement is a no-op. On the one hand that represents a missed opportunity for revenue. It also means a missed opportunity to perhaps accelerate the growth of that company beyond what it has managed to achieve on its own.