> The important bit is China's massive spike between 30 & 57. What's the plan for the next 30 years?

I don't know, but again, given the fertility rate in the UK, and the recent clamp on immigration, I don't see how they could fare better.

> It's not just about raw numbers, but how well those immigrants integrate into the target country

Right, but you have not been able to justify either. If there are no metrics on how "well" immigrants integrate, then the point is moot.