I don't understand this. Rather than pay a (yet to be proposed)
higher capital gains tax, Americans would pull their money out of the markets
right now and put it into what? Is the plan to make no capital gains in the future?!

Plenty ways for plenty people on the border range of having to pay something like that (if it passes which is probably 50-50 I'd say) to think of other ways of making money.
A crypto or stock whale could draw money out now and invest in property now with guaranteed rental income rather than Biden hurting an entire portfolio of assets in one go etc.
The closer taxes get to the 50% threshold, the more people will just think I'm not going to bother and wait for a gain that may or may not happen if I have to pay that much.
Seems logical to me.