You may disagree, but “nonsense” sounds a bit excessive. An Italian or Spanish software developer earning 30K euros per annum would likely go to the US if they were offered an American salary. Many of those who went to the UK or to Germany (me included) would have crossed the Atlantic for the right offer.
I agree with you that inexperienced (lower paid) or young developers might find the US attractive if there weren't visa issues. The older better paid ones, maybe with families even, would probably still choose the social safety of Europe, the mix of people and ideas, the density of interesting places, cultural events, etc.
I mainly reflected on the current trend I observered. Did not consider a sudden change in the US visa policy.
A half decent flat in London costs in excess of 2000£ per month. Where “decent” means you don’t deal with pests, walls are not made of paper and your neighbour going to the toilet doesn’t cause the whole building to shake.
Universal healthcare varies a lot in Europe, for instance in the UK it is rather limited compared to continental Europe, even compared to Southern Europe. In the UK you would need a health insurance costing ~200-300£pm to enjoy the same quality of care of places like Italy or Germany.
On the other hand a good developer in the US may earn enough to retire in 5 years, 500K$ per annum are not totally unheard of. In Europe if you are very very good, you may get slightly above 100K£, but it’s still extremely rare and you will end up paying half of that in taxes.
Depending on what we mean by en mass, because Europe is not the third world. But I think it would have an impact on the European market(s) for software developers (as in increased scarcity, higher salaries, ecc…)