> If we decide we want to tax that, it seems like the only means we'd have available is to force him to give up shares in his company.

I assume you mean because he'd have to liquidate profitable shares in order to cover his taxes?

If the concern is the attachment of profit-sharing to control, we already have profit-sharing non-voting shares and non-profit-sharing voting shares. The necessary division for maintaining ownership while preventing infinite untaxed wealth accumulation already exists.

So it seems to me, then, that taxing unrealized gains only equates to giving up shares in your company in the same way that taxing income equates to giving up shares in your bank account. Do the taxes cost you future return/interest? Yes. And?