You’re envisioning a swift transition to a sort of perfect global preference equilibrium. I don’t think that’s remotely likely.
Assume all borders became open tomorrow. (I don’t think there’s a scenario short of the total dissolution of all nation-states into a single world government in which “open borders” could ever mean literally zero customs and immigration apparatus, just walk in, so to keep “open borders” a remotely plausible and coherent concept even just at a policy thought experiment level, I’m defining it as the scenario in which it is feasible for anyone to move anywhere. That is, residency with a path to citizenship in any country is available to the average person who’s willing to jump through manageable hoops, e.g., long waiting periods for full citizenship/social benefit eligibility, basic language skills testing for same, adequate security screening. This is vastly different from current-world reality, and I’m very comfortable calling it an open borders scenario.)
Huge numbers of people will not instantly decide to just up and move to another country simply because they like the idea.
Moving to a different country is undertaking a massive, pervasive, whole-life transition. It is not a thing you just up and do in the spirit of comparison shopping like buying a Honda when you’ve always had Fords.
Emigration is a huge, frightening, difficult, ~alienating~ undertaking for many, many more reasons than just the thorny logistics of obtaining legal residency status.
Even the most ~rootless~ technocratic globalist universalist types generally have families, local ties, careers premised on the industrial and credentialing infrastructure of the country they already live in, comfort zones, etc, and most won’t just up and leave everything they know and move far away to a strange place without a lot of weighing and considering. And if you’re concerned about immigrants showing up here with values inimical to Western culture or whatever (cf the big chunk of thread already cut off, readers with a strong stomach should see the notes), those aren’t really the people you’re worried about. You’re worried about people who have very strong rootedness in their own tribal and community norms, where those norms differ from ours. Those sorts of people won’t all just instantly up and flock here in droves for reasons much less compelling than “my home city has just been bombed to smithereens” or “my government is currently rounding up people of my [race, sexuality, religion, etc] and shooting them.” A fat pay rise might be enough to swing the open and curious globalists; it’s not going to be a good enough reason for the superrooted.
I of course speak as someone who is interested in proactive/elective emigration. But I didn’t acquire that interest in a glib or facile way, or merely on the basis of superficial preferences in climate or architecture, as much as I like talking about those sorts of things. I have experience living abroad as an adult, and found it challenging, despite my natively low susceptibility to loneliness. I also have already immigrated once, with my family to the U.S. as a small child, and even though we had every advantage as immigrants — my father is a neurologist and ours was a thoroughly considered, planned, self-funded, legal and aboveboard move from another anglophone country to an America in a welcoming post-bicentennial “melting pot” mood — the psychological toll on all of us
was meaningful, deep, and lasting. I don’t trivialize what emigration involves. I don’t think that most people do. (It was worst for me as an introverted child and my mother as an extroverted adult; my brother as an extrovert starting kindergarten assimilated far more easily into U.S. school culture, and my father as an introvert took less damage from social uprooting and precipitous loss of regular interaction with cotribalists. As an adult, I figure I’m relatively temperamentally advantaged for weathering the stresses, and I have good language skills, but I expect to go through some very tough times during the first few years if I do manage to move. This shit is hard. It’s not a Consumer Reports clipboard shopping kind of decision.)