Others tend to become Generation Ships of the founder culture. I'd put the Well in that category, as the userbase still seems to be strongly focused on the original cohort. Some fresh blood is necessary.

Actually: the idea of a university as an intentional community that tends to survive over time has occurred to me. Not fully assured, but there are an impressive number of centuries-old instances.

And, continuing that thought: YC emulates a critical element of that dynamic, in having regular incoming classes.