Countries like India, Nigeria, etc won't crest their growth for decades and in that time they will add another 1-2 billion people to the planet. Even after the cresting going below replacement will take another generation or two. So for at least the next hundred years there will be plenty of people in the world to import if you want to keep your population pyramid from changing.
That is all aside the fact that its well established now that demographics are shifting even in limited population growth scenarios. Urban migration continues because the availability of productive labor anywhere except at the highest concentrations of human capital continue to diminish. There is no more gold rush of raw resources to establish whole states over and all the industry we have for resource extraction is getting automated away.
In the next hundred years we should see the gradual centralization of human populations for purposes of survival. And as the available job pool shrinks, until the jump is made to a UBI / socialist / post-scarcity model in the future the increasing pressure basic necessities have on people will keep forcing the cost of labor to stay stagnant as it has been for 40 years and thus inflation will pressure people into living more efficiently.
Right now having a car and commuting 4 hours a day from a 3000 square foot house with a yard you need to maintain, where in part you need to pay for all the roads you traverse, all the infrastructure wired to your house, the services required (fire, medical, police) is insanely expensive and insanely inefficient compared to people living in condos, apartments, etc with public transit in a proper metro. Automation will keep gnawing away at median disposable income until the median is forced to forgo luxuries like split housing.
Those transitions and shifts will require legislative support, which given the fact those shifts will be made because of outdated state policy probably doesn't bode well for it being a smooth migration.