Replacement migration doesn't really have to do with the absolute numbers in the world but admitting immigrants from countries that have well above replacement rate. The world population is so large that the countries who desire increased population have a loooooong runway.
The only problem, exemplified by the Windrush Generation in the UK, is the rampant racism towards the people who are immigrating to help you.
The error is in the modern western belief that human breeds are equivalent modulo æsthetics and training. East Asia is a rising star as the west commits altruistic demographic suicide!
Yeah, and we might have stood a chance at constructive reform had so much of the population not fallen for the siren song of fascism, itself just a different lazy option offered by the elites. Same old story - they turn the screws of extraction as long as they can get away with it, and when the jig is up they contain the ire by making sure it's directed at superficially different individuals.