I'm not aware that alt-right is genuinely worried about population collapse in China or Japan. Advocating for bigger population has nothing to do with right or left, or are you telling me Matthew Yglesias is alt-right too?
In his new book, political reporter Matthew Yglesias imagines a future America that's revitalized itself by tripling its population.
www.wired.com
And I too have read plenty of SF, then and now, Yglesias is correct in that there're way more SF with overpopulation theme than population collapse theme (the natural kind we're having, plenty of SF about humanity getting wiped out by disaster etc, that doesn't count obviously). This is especially true in the 60s and 70s, probably thanks to Club of Rome.
If you guys can take off your anti-Musk woke glasses, it should be fairly easy to see why he's somewhat obsessed with population, and it has nothing to do with politics.