The assumption that stock prices don't matter, because company ABC doesn't necessarily depend on it 'for now' - is just wrong I'm afraid.
If Apple crashed to 1 cent it would gut the entire market. Even just that crash alone, the amount of money wiped out, retiree savings etc., other companies would be valued lower and then the mass selloff would crush them as well.
Apple may not need to 'raise money now' but it very well could in the future - and - every company is somewhat of a proxy for every other company.
If there is no ROI, there is no investment, and there is no economy outside the government, it's that simple.
Taxes on unrealized gains are a separate thing, and probably a bad idea - just contemplate that they would have to be paired with tax-sheilds on unrealized losses as well. Due to speculation, it would open up the door to all sorts of shenanigans.
It's just a bad idea all around.
Elon Musk is a 'paper zillionaire' that's very, very different than someone with a zillion in the bank.
There are probably some very boring, old, already established ideas for increasing taxes on the ultra-wealthy that would probably work very well. Including getting rid of loopholes etc..