>but it's a problem easily solved by just letting in more immigrants.

right on cue. it's a Pavlovian response at this point, isn't it?

now please provide one example of a country that had "easily solved" its demographic decline via immigration.

Not decline, but Australia considered its population too small after the Second World War and very successfully integrated immigrants from an increasing diversity of source countries: https://en.wikipedia.org/wiki/Post-war_immigration_to_Austra...

Japan is already accepting immigrants! It's right there in the article I linked! It's just a question of raising the quota slightly.

It's hardly going to solve every problem, it will keep restaurants from closing.