Maybe I am wrong and there are visa holders feeling absolute confidence at the moment, but I would like to hear why from even one before considering your "a few people" categorization as anything but spin.
It's so far out of line of reason to believe that the US is going to basically end all - or any meaningful portion of - legal immigration, as to be nearly impossible to reply to. Where to start?
The US will switch, if anything, to a merit based system. Canada already uses just such a system. Nobody seems to be outraged about it, or proclaiming the end of Canada because of it. There's far too much emotion in this thread.
But all Europeans that would like to get visa are on hold right now. People that already have one are in limbo, worrying about how the coming ruling will affect them.
If you're able to interact with the smartest people in the world - and not just your own country - you end up forming more effective and larger companies. Which in turn create jobs.
If you shut out your country to the rest of the world, the next Google will be founded somewhere else.
And I'll dispute your claim that the US has enough talent. There was a submission on HN recently about the top deep learning papers. Go through it. Count the number of American-born people. Geoff Hinton? Yann LeCun? Russ Salakhutdinov? Demis Hassibis (of Deep Mind)? Virtually everyone on there was born somewhere else. They're not Americans, but they're mostly working for American companies and contributing their talent to YOUR economy.
[0] https://github.com/terryum/awesome-deep-learning-papers/blob...
But so what? That doesn't demonstrate that the US doesn't have enough talent. It just shows that some people from out of the country were working on something. Maybe if the situation was different then that paper could be exclusively written be Americans. The majority of the worlds most intelligent people reside elsewhere, but the US has nearly 400 million people. It's fine with or without immigrants. Japan is fine isn't it?
Many papers are written exclusively by Americans, but they're often not the best. That's what you're going to have to be content with, if you shut off immigration. Being okay, but not the best.
Imagine that those researchers couldn't go to the US. Imagine instead of Google DeepMind beating Lee Sedol it was a Canadian or Chinese company. Imagine if the first company to roll out mass self-driving cars would be Didi instead of Tesla. It's an inevitability, once all the top minds in the world go to companies outside the US. And as I said, it's not horrible for America, the country will surivive and do okay, but there'll be jobs lost and innovation will go somewhere else.
Reading the news, I would not bet too much money on that either.
Most likely not next year.
In Germany and many other European countries, when the government resigns (or the government is mistrusted by parliament), there is the possibility of immediate new elections.
Is this totally impossible in the US?
The system we have here is quite different from the parliamentary system you may be more familiar with.
For all the reverence the 'founding fathers' get they could have done a much better job setting things up. America got rid of royalty, then brought it back in by electing them, and not in a ceremonial function either. It's a huge problem (that, and the 'two party system').
Technically the two party system could change but things are set up in such a way that no third party will ever gather enough votes even if you were to say pit two un-electables against each other.
Presidential elections happen on schedule. If Trump should leave that gives you Pence, if Pence should leave that will give you Ryan. And likely by that time it will be 2020 and you'll have those new elections but until then you'll be stuck with a Republican as president, like it or not.