Most important is that PG's definition of a startup (or Fred Wilson's or Steve Blanks or pmarcas or Doug McClure's etc.) is to serve a purpose of what is good for them in their business model which is to make money off of people who take chances with their time hoping for a certain outcome.
It's not about what is good for any particular person or for society. Guess what? That's not what business is about either. And YC is a business. It's not on a mission to help the world although that could be a by product of a successful investment.
That's fine as we all tend to do what is in our best interest (in varying degrees of course), but it needs to be recognized and considered when one makes a decision to go down that path of which the person taking the journey is the one who has something to loose (time or impact on family).
Possibly the parent poster is jesting or perhaps they didn't realize this before, but it is quite obvious that what people write is self serving in so many ways. PG is no different.
Notice also that PG didn't exactly do anything "ambitious" with his time. He choose a path of certainty (Harvard and YC was certainly not "bet the ranch" in any way it's an excellent idea that in retrospect anyone can see had an excellent chance of working with a smallish downside). Viaweb was not "ambitious" either. I was around when that was founded and it was fairly obvious small business needed online stores and there were multiple companies doing the same thing.
Before you downvote, this is not in any way to rain on Paul's parade or take away any of his achievements at all. Or to try and make a point that there is anything wrong with what he is saying vs. what he has done.
There are always the "to be sure phrases" sprinkled about. PG does this quite early (in the fourth paragraph) by stating "because most startups fail." which of course is slipped in there the same way financial firms say "past performance is no guarantee of future gains" in their advertising (while taking out expensive full page ads in order to tout their past performance - at least they did back in the day..)