I've heard an opinion that having less people leads to a technological regression, because some things to create/research are so expensive that they become profitable and functioning only at the world's scale

E.g. China is too small to have an isolated closed market for a competitive and efficient semiconductors manufactoring

No it is just that capitalism is a cancer. Nobody has actually cracked the code on how capitalism can work without perpetual growth.

Leaders are looking at Japan and they are panicking. Fascists are demanding more white babies.

FYI, socialism also cannot work without infinite growth.

Large scale socialism maybe. It seems to work fine in small hunter gatherer bands.

Socialism works fine without infinite growth.

The problem isn't that it requires infinite growth, the problem is that as the scale increases the drag effect of those who consume more resources than they contribute grows faster than resource availability grows. There's a maximum size limit that's based on things like cultural sense of responsibility.

And yet somehow those people are never advocating for leading by example.