after X billions of dollars of funding, there's a point where a startup's success is pretty much guaranteed

I don't agree that this is how it works. Abundant funding is very far from a guarantee of success. There may be a bit of selection bias going on here. Most companies that VCs fund fail, but you only really hear about the small sliver that succeeds.

Further, it really does happen that being overly funded leads to failure in and of itself.