Several EU countries have already closed or will be closing their sovereign borders in the next 24 hours. They can do that. They are doing it.

He did it in an ineffective way. US nationals were returning from the EU without any screening. And it was way too late. All a bunch of xenophobia theater.

Though I agree with your criticism in broad strokes, this is also a numbers game. If it reduced the number of infected people entering then it was useful. It doesn't have to completely eliminate it to be worth doing.

There are legal and logistical challenges in preventing US residents from entering or imposing quarantine.

Would strong or (more importantly) earlier responses have been good? Absolutely! But a weak and late response is much better than an even later response (even if the later response is stronger).

By the numbers we should prefer a non-US-resident ban-- even without screening/quarantine-- on day 20 than a everyone-ban on day 40.