I think it's hard to compare Japan with the US or UK or other countries because unlike those countries, Japan can't turn on what I'll call an immigration valve and just flat-out import people to grow the economy.
It's not impossible, but it would take a lot of time and Japanese culture isn't exactly known for xenophilia.
I doubt that Japan will be going in that direction any time soon, but if they wanted to, they could.
Tokyo is crowded, however.