So in your book Chinese and Japanese are whites, got it...
Elon is making a rookie mistake and assuming a straight line from now. If you look at the population charts for various countries you will see all sorts of variations year to year in the number of people. Just because the birthrate is very low now doesn't mean it will be in 10 years.
Japan can afford to shed some population. I believe it's the most densely populated archipelago on Earth. If it isn't #1, it' close.
When populations shrink, it's rough economically while the shrinking is going on, but over the long haul the country or region does much better. When the population is large, there is more population for jobs and pay is low. With low pay, people on the bottom of the ladder can't make enough to get any skills and move up the ladder.
The population can stratify into a static system with the people on the top having no incentive to do anything but rake in money and the people at the bottom stuck in poverty. The idiots get rewarded for just being born into the right family and the people with the potential to do something great never get the opportunity to get off the first wrung.
A meritocracy is more likely when there is a labor shortage. The people on the bottom who are motivated can work harder and make more money. If they can't get a better education, their children can.
That is what made the US the land of opportunity. Virtually all my ancestors were poor in their home countries and came here. Their children all did much better than they could have back home. There were barriers for some, but in many cases they still did better than back home. The people who did emigrate were often the more ambitious and they were going to a country with a perpetual labor shortage in the 19th and early 20th century.
I grew up in an emigrant community and saw the same dynamic playing out with Hispanic and East Asian children at my schools. My partner's father came to the US in 1931 as a child. His generation lived by modest means, but quite a few of their children have professional degrees. My partner and a handful of her cousins competed for who had the best education in the family.
There are no underpopulated frontiers for the ambitious to go today, but population decline will offer the same opportunities that the opening of North America offered. After the Black Death labor suddenly became a rare commodity in Europe and after 1000 years of economic stagnation, the continent exploded with new ideas and technology. Part of it was a shift in attitude among the survivors, but the new labor shortage gave an opening to the ambitious commoners to move up the economic ladder and they did turning Europe into the richest continent on Earth. There were downsides to that, the remaining indigenous peoples of several continents can attest, but it did propel the world into the modern age.