You obviously downscale the economy with a population that doesn't sustain or require growth.
Do you have a concrete policy proposal for a country like today's Japan?
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).
the basics of econ, growth isn't even mentioned when it comes to capitalism.