"For non Americans, is there a 1% chance of (say) a ... punishing withholding tax on sales? Would a US brokerage one day be asked/forced to apply some rules that might not be lawful under US law? Note that there is already a rule that if a non-US-person sells shares of a listed LLP, the entire sale proceeds (not the profit) are subject to 10% withholding tax which is (in my case) non recoverable. Buy $100 worth of Sunoco, sell at $105, lose $5.50."

As a non-American this is an interesting thought experiment.

I could see how a US politician might dress this up to a domestic audience, saying that "furriners" would now be paying big taxes and no longer "taking advantage" of the US.

I suspect the first-order outcome would be immense selling pressure on Wall Street, as the non-US investors sold. That would dramatically cut share prices. I'm not sure how corporate America would like that, let alone Main Street America watching the value of their 401k's tumble.

I suspect the second-order outcome would be for the larger US listed companies to dual-list onto an offshore exchange, so that non-US investors could trade their shares without incurring the withholding tax. Make London Great Again. To be fair Trump might get off on cutting Wall Street off at the knees, because he seems to harbour resentments at the NY establishment.

And I suspect the third-order outcome would be for the USA's position as the global default currency to fall even lower.

I also suspect the withholding tax would end up generating very little tax revenue. It's the challenge with taxes and tariffs ... set them too high and you deter the activity you're hoping to tax and so end up collecting no tax income. You can either use them to be punitive or to collect revenue, but it's hard to do both.