No one needs mass immigration. Especially those with high living standards and "low" birth rates. The position you're taking is an outright lie and I can't fathom why it's constantly being perpetuated.
I was worried that by "downscale" you meant to let people die, so I had to clarify.
As far as the claim that capitalism doesn't require infinite growth: that doesn't seem mathematically possible, ans it's certainly an unorthodox claim. I'd say that puts the burden on you to provide evidence (or, at the very least, explain).
If birth rates are low, how does your economy function with many older workers who can't/won't do labor like construction?
Why is Japan opening itself up for immigration specifically to fill jobs? What other solution should they have chosen?