I feel a bit awkward being the one to bring this up, but the idea is that we're supposed to give the startups useful advice. The need for that doesn't necessarily go away when e.g. you raise series A. I often give advice to post series A YC startups that they seem to think is useful.
My impressions are made from afar, but it is that YC is a high intensity immersion/sprint for founders to catapult themselves to the next level while having the support of experienced advisors and colleagues in the same situation around them. In other words, help shape and catalyze founders to become great with anything they touch (and recognize what to avoid). Strictly in the context of YC (or TechStars, Excelerate, etc.) it seems like that becomes lost when working with founders at a higher experience level.
There are a variety of "next levels" one can catapult oneself to. In the last batch we had one pair of founders who'd previously been CTO and VP of engineering at a public company, and another pair who were 19.
It's an interesting question where the limit of our utility is. As I said in another thread recently, I'm pretty sure we don't bump up against it in practice, because once startups raise sufficient money, their investors wouldn't let them do YC even if it would be a net win for them.
