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?!
We don't have a capital gains tax, but as far as I understand you'd pay the current tax if you withdraw now.
So, if you gained $100 and tax is 20%, you owe $20 in taxes now if you realize the gains. Then you start from scratch with no gains, and taxes go up to 40% only on your future gains (not on your past gains).
I never really got this though: if the tax is fixed, why would you want to postpone paying the tax, if you have to pay it eventually anyway? Unless you're hoping for the tax to go down.
If the taxes are big enough, you don't pay the taxes.
If the taxes are big enough you can find ways to avoid taxation.
Only poor people pay taxes.