I mean, is it not possible to sell a small % of your shares to cover the tax?

Moreover, you could create a multi-tier share structure where controlling ownership belongs to the founder whereas financial interest in the business belongs to the holders of other equity flavors, which is fairly common (e.g. Meta, Snap are structured this way).

Then the founder can [choose to] sell those cash-flow-stocks while holding on to the ownership stock.

It's utterly hyperbolic to suggest that a wealth tax would kill privately-controlled corporations.