This is an oversimplification, they can change incentives, and sometimes increase investment.
> lead to less activity
I do agree money will be divested from the US as they become more and more expensive to deal with (leading to "less activity"), and like I said this will rechannel the economy between the rest of the world. The trade-off is that the US becomes a manufacturer and exporter again (leading to "replaced activity"), some manufacturing capability is duplicated (leading to "more activity" though redundant/less productive), and the currency devalues.
But I'll admit I'm well out of my depth here, and I'm being booed off the stage. All the same, I don't think I'm wrong here. Protectionism isn't new, and lots of countries do it, it's just novel that the world's largest economy and bastion of free trade is doing it to such an extreme.