> even just 20% of those leaving would mean every single big tech company would be in immense trouble

I'm not so sure.

I think it would play out like this:

1. 20% H1Bs leave; 2. Those migrants are now in countries of origin, looking for work; 3. Many of the big US tech companies will already have offices in those countries, and those that don't can make new offices if they wanted to; 4. many, but likely not all, of those employees are now working for the same employer (or close enough), just in a different jurisdiction; 5. as none of these employees are physically in US hotspots, all the other stuff that happened in those hotspots because of big tech pay, suffers, conversely all the stuff which was suppressed because of those wages may (possibly) return; 6. two of the things that go down are the number of people transitioning from temporary visa to citizenship, and the available talent pool for the local-to-those-places startup and VC scenes.

Certainly a lot of them do. It's also true that having a large portion of them leave will just mean that the company will have to replace them with someone who will require a higher wage, and won't have any issue leaving if the workplace culture degrades.