I recall Jessica Livingston defining "died" as "founders stopped working on it", which I think subscribes to the while there's life there's hope school of optimism (which is very appropriate in my opinion, especially when many startups only become successful after they've completely redefined their business. Is it "the same" startup? I dunno, but it's the same founders, and that's what YC funds.)

But it also means that "I'm not dead yet" doesn't necessarily imply "successful". I think relatively few YCs have been bought out/received further funding; and I would guess for most of them, to be realistic, it's simply too early to tell, because of the above.