As I said, elsewhere, I'm okay with the welfare state collapsing (it will, anyway, eventually), but I think most of the people promoting open borders don't realize that it will result in all welfare being privatized and largely disappearing.
Well, I am being charitable. Some of them are probably promoting open borders because they know it'll collapse the welfare state rapidly and eventually lead to anarchy. A lot of people are eager to build on some ashes, so they're arsonists.
I don't think welfare systems typically collapse. Rome's grain dole lasted centuries. Under stress, eligibility requirements are tightened, so the recipients have to work harder to demonstrate political reliability and ideological correctness.
Well, it would collapse down to a new equilibrium, at least. From "Here is your mobile phone, fully-furnished apartment, preloaded debit card, free medical, dental and university, and now we'll fly in your 7 closest friends and relatives, too" to "three hots and a cot".
It would have to keep getting less generous and more restricted, the longer the border was open.
True. Requirements can and probably will at some point escalate to proof-of-violence against regime-citical symbols and persons. That and an expected level of (self-) loathing, poor self-care and emotional disregulation. You know, like they already select for...
Ideological tests wouldn't work, as the open borders promoters are usually socialists and their test would be something like "Do you think we should let all poor people move here and get free stuff, so we can be nice and friends?" That wouldn't limit the number of welfare recipients, because that's the target group.