Why would it matter if they didn't? All they would have if they held it is more money on top of the gobs they already had.

When you have that kind of money, if you want to grow it the surer strategy is to invest in lots of stuff, not keep it all sunk in your previous employer. It's probably more fun too.

And beyond a certain point that stock and the accompanying valuation in AMZ probably isn't so gratifying in itself, and unless one has a juvenile obsession with out net-worthing others, you need to find something more personally meaningful to do with it, whether that is start a new industry (Elon Musk) or address pressing global health issues (Bill Gates). It sounds like the GP has spent some of his funding free software.

Put into perspective that the dot boom happened around 1999-2001 and the dot bomb set in hard by 2003. Between 1996 and through that roller coaster people went through a lot, and since then there have now been two financial crises in the US, one ongoing.

It probably doesn't feel good to be asked this question. I say this as an early employee of three startups.

90% of startups fail, in Las Vegas you have a 14% chance to win now make your choice. I sold my stocks (not Amazon but a very early employee at a small company that still exist and somewhat profitable) as soon as I could and ended up selling at an all time high so YMMV