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?
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.