$90M is definitely enough to be more than comfortable the rest of your life. But a $5B payout would have meant they could start a VC firm, invest in the next several generations of startups, partially self-fund something ambitious like a Space-X, start funded non-profits, etc.
...yes? I don't really see what's so absurd about that idea.
It also seems more than a little disingenuous to suggest that the founders were the only ones that created that $20B in value. They didn't single handedly create the apps, the marketing platform, drive the cars, etc. etc.
https://en.wikipedia.org/wiki/Y_Combinator
> In 2009, Sequoia Capital led the $2 million investment round into an entity of Y Combinator which would allow the company to invest in approximately 60 companies a year as opposed to their previous 40 companies a year. The following year, Sequoia led a $8.25 million funding round for Y Combinator to further increase the number of startups the company could fund.
I think they'll be OK if this is their goal.