This is an absolutely valid point, thanks for raising it. The US has (partially) addressed it by being an attractive enough economy that brings in young immigrants with "fire in the belly". These people (I am one of them), stay long enough to add economic value, and, crucially, have children who are integrated into the broader society without giving up on their native cultures.
Japan has historically restricted immigration into what was a dynamic economy, which stagnated over time.
Japan has historically restricted immigration into what was a dynamic economy, which stagnated over time.