Income taxes are the ones that apply to income. Other taxes apply to other things, e.g. property taxes apply to property and sales taxes apply to sales.

So wealth tax then! It’s really just a question of finding metrics and then sending people bills to force them to liquidate or whatever

And what if the stock valuation crashes before they can pay the bill? On hook for money they don't have?

Seems almost impossible to implement

There are already things in tax law for this kind of stuff. You can write off the losses. If you're rich enough to be hit by this stuff, you're rich enough for an accountant and lawyer to help out with this kind of thing.