Japan is not an apt demographic corollary for the USA. The census recently came out and found that the number of Asian and Hispanic residents of the USA has increased (~20-30%) hugely since 10 years ago. These are younger populations with higher birth-rates than "White" people, whose percentile share of the US is heading downwards.

Japan, on the other hand, is xenophobic and has discouraged immigration very heavily. Combine that xenophobia with historical matters: sneak-attacking an industrial powerhouse in 1941 in of the most ill-advised, terrible wars, losing repute by massacring hundreds of thousands of civilians. Meanwhile thousands upon thousands of their own best young men and civilians were killed by the vastly superior man-power and industrial might of the US. Japan was hobbled by WW2 and has never fully recovered, consider the greatest catastrophes of their history were only 80 years ago still, namely losing a generation of youth, their cities being fire-bombed, their savings being depleted for phony war bonds, and being the only country to ever be nuked. Japan is simply not a good demographic comparator for the USA.