Brain drain is really not an issue at all in Western Europe. Sure, some people move to SV, but the majority likes sane working hours, streets without poo, and schools without metal detectors at the entrance.
Anecdotally, the amount of European programmers I know that moved to the US slightly exceeds the amount of American programmers that moved to Europe. I strongly doubt a different visa system would impact that significantly.
I don't see any brain drain from Belgium to US actually.
Everyone knows US pays more, but you are left in the dirt when something goes wrong.
People stay mostly here since there is work enough, friend & fault is here, travel between countries is easy, pay isn't bad and you get a decent healthcare.
Some people work in the US for a couple of years, but all the ones I know come back ( outside IT I have seen some friends going to the US)
I meant more that the loss of US power is unfortunate (and was being a little glib, obviously upsides and downsides to the US not being the only superpower anymore) - I would love if more Dutch came to the US, maybe SF would finally get some bike lanes
[1] what a name