If you have an idea and a management team that's good enough to make it with the help of YC, it's hard to believe you couldn't get the funding and make the connections that you need to succeed in another way.
I don't know a whole lot about the success of YC companies, but it looks like Reddit is their big success so far. Reddit sold for a reported 65 million (according to The Google), and they needed relatively little additional funding after the seed money. So let's say YC retained 3% of the company, they make 2 million which is a 100x return on their investment. Wow, I just did the math now...that's pretty impressive.
Update: A new search on The Google (link) suggests that the purchase price may have been much lower. Proponents of the "50-million my ass" school of thought are suggesting 5 million is closer to the mark. At 5 million, YC gets a 10x return on investment which is what all VC money is shooting for among successful (non-broke) companies.
Larger point, if you're in it to win it you should be able to pull it off with or without YC. Sure YC connections and advice are tremendously valueable...but 5x to 20x better than the other smart VC/angel/seed money out there? No way!
