If current rates continue, which is a big assumption, then Japan will have 106M people by 2050 and 75M by 2100, which was roughly the population before WW2. Japan is already one of the most densely populated areas in the world, so further population growth may not be the greatest idea. They already have good standards of living, great infrastructure, high societal trust, low crime, a competent government, etc. Europe's strategy of importing millions of low skill foreigners who are mostly dependent on welfare and have destabilized their societies is also probably not a great idea either.

These population projections also ignore the many factors that may change. As the population declines, more opportunities may open for young people, leading to increased family formation. The culture may evolve in response the lopsided demographic pyramid, causing increases in the fertility rate. People are already predicting AGI in the next couple decades, which will render most people unnecessary anyway. These classical analyses around demographics will probably become increasingly obsolete.