On top of this western productivity is linked to low birth rates enabling a larger percentage of the population to work. So low birth rates may be required for a wealthy country rather than being just the result of a wealthy country.
Of course that doesn’t pay for significant medical care etc, but I also ignored economic growth. By the time India has US style age demographic breakdown things should look significantly better.
With accelerating automation, most people are going to have to depend on a much smaller set of people/industries. A shrinking population is not bad given we make plans for the future.
Mobility is, in the modern world, as much a political argument as an economic one.
A country can be "highly mobile" with flat GDP and limited income growth opportunity for the individual. That is just how the math works out. Social mobility is a term-of-art that does not mean what most people intuit it to mean.
Consider healthcare, usually older people need it more than younger people. If you have a higher proportion of older people in a population compared to younger people, you have a larger proportion of the associated costs being carried by a smaller proportion of the population, this means decreasing healthcare quality, increasing the pool of people paying in by increasing retirement age or increasing taxes. None of which are particularly good.
By far in poor countries healthcare is much more affordable than in rich countries. I think one of the main outcomes of rich countries is just increased costs for everything.
Would you rather old people be dying of starvation/due to lack of healthcare because the country isn't rich enough to support them fully than living in old age homes?
How about having the goals of developing healthcare instead of "being rich"
Look at how the poor in India live. "Getting rich" does not mean becoming multimillionaire, it just means reaching a developed country's standard of living.
This is most visible in America’s response to COVID but also in Prop 13 etc.