This is such a privileged take. Outside of internet debates, the vast vast majority of people still see the US as the best place to immigrate to. I'm originally from north Africa, and America is still seen as a better place to be than Europe, by far. Going to Europe can mean years long unemployment, racism, low wages and is only considered because it's more accessible than the US.
In terms of opportunities for an immigrant, America is just in another league. Maybe tech workers don't see the difference, especially if they are from another western country, but that's not representative.
Also, I completely disagree that there wouldn't be a huge wave of immigration if ther borders open. Yes, moving is hard. But it's a complete no brainer when you have almost 0 prospects and opportunities in the place you currently live. They wouldn't be moving to just get a higher salary or a better career, they'd do it because they could barely even earn a living.
It's hard to imagine for westerners, and I think it's coming from a position of privilege where you can't really imagine things being so bad that you'd just move without hesitation at any opportunity. But that's the current situation for hundreds of millions of people. Wouldn't you jump on the opportunity to move if you lived somewhere where getting a McDonald's job isn't easy and youth unemployment can reach 50%?