Earlier quoted context omitted.
I don't understand why a population decrease should be resulting in "huge problems". I am sure it's possible to maintain or even increase happiness and relative wealth while growing smaller. For earth in total, it would be great to see a smaller number of humans.
who pays for all the old people?
So far, we have increased our lifespans up to 85 or even 90 years, but our healthspans are significantly shorter and those last 10-15 years are spent in state of partial or complete dependency on others.
Unless we can do a lot more in the longevity field and stretch healthspan of old people significantly, we won't be able to meet the total demand for workforce in care.