"We still aren't at the point where we could decrease the population of USA...using air travel. There was never a time when it would be 'a reason' to pursue air travel tech. It still is not a time to pursue air travel tech as a means of decreasing country-sized populations. It is not a reason to pursue space tech either."

It doesn't really have anything to do with air travel- no country has significantly reduced it's population by moving by foot, ship, train or car unless it involves fleeing from a war or widespread disaster, and even then it's rare at rates of say 30% of population within a year.

But nevertheless your point is taken. The immigration from Earth would probably be limited by a rate which is some percentage per year of existing population living in space and probably a departure rate from a country is limited to a small percentage [less than 10%] of it's current population.
And that is due to the social disruption large scale departure and immigration in new area would cause.

So, less than a decade is probably too rapid and instead something like 2 decades would be fastest it could be- and at this rate there would still be plenty of social disruption that would need to administered competently and efficiently.

One if fastest migration may be when Pakistan separated from India- that might make good case study, but I don't know of the specific numbers, etc.