Found a mistake near the bottom: Justin.tv is either dead or acquired, however you want to interpret the change into Twitch.
A lot of old companies are Dead or Exited. Someone should do statistics on this data.
Data is not fully accurate, especially in the many cases where a YC company had pivoted and changed their name.
Also, there's a third startup possible startup state that's relevant but impossible to measure: Zombie, where the startup is alive but will basically never exit.
What about "successful private company"?
Doesn't help the investors though, unless they pay a dividend.
What stops YC companies from doing this? You get your 150k from YC, you're making a few million a year in profit, and instead of killing yourself trying to make billions, you say "good enough" and hand the reigns over and let the company keep doing its thing conservatively. Pretty sure that's what I would do.