Owning shares isn't only about wealth, it's also about control. The kinds of people that create successful startups aren't likely, as a rule, to give them up without a fight. By the time you've divested 50% of your shares in the company you founded it's no longer your company, and yet with 50% in one company you're still nowhere near sufficiently diversified to be able to count on those average market returns.

no, you don't divest and lose control, because you can pay taxes and diversify without selling shares (incidentally, this is similar in shape to a credit default swap). even so, you don't need 50% of the shares to control a company. in both of these instances, there are many more ways than one to skin a cat.

> no, only the most obstinate would not diversify their holdings once they reach tens of millions, let alone billions.

Just do the math, you need to divest a significant percentage of your own holdings in order to re-invest it into diversified portfolios sufficient to pay a wealth tax. The vast majority of founder paper-billionaires haven't come remotely close to doing this.