I don’t get the math. 2 kids = 1 parent each; 1 kid = 2 parents. If the parent survives beyond age 50 and has no nestegg, and if they do have a nestegg, allowing that parents, not the grandchildren, support the parent’s parent, aka grandparent. Add a spouse and you still get 1 parent each or less if the spouse comes from a larger family.
Neither calculation is correct.
The simplified case is that a birth rate of 2.0 children per woman (on average) results in a steady population. Ie., each generation is the same in numbers as the last, so, each individual on average will end up with one elderly person to support, for however long that elderly person is essentially non-productive or has run out their nest egg, but is hanging in there.
If the birth rate is 0.5, then each generation is 1/2 the numbers of the last, and each individual ends up with, on average, two elderly persons to support.
This will be countered somewhat by less children to support, in the 2nd case, however, on average, children are much healthier and cheaper to support than elders.
The problem of a low birthrate is compounded somewhat if generations get longer, and ditto for longer life expectancy, if productive life does not increase by the same amount: If on average parents of age 50+ are still supporting their kids (for or in college, or whatever) AND also supporting elders, for the average person, their standard of life will go in the crapper. (I have personal experience with this!)
Also consider that these societies almost invariably are quite generous to a large portion of their society that are of productive age, but are not productive. This increases pressure on the birthrate (people can’t afford kids). Taken all together, these societies then decide they need to import labor: The country may survive economically for a while (esp. if the US is providing most of their defense!), but once they start importing en masse cultures who wish to impose even greater negatives, the situation may become unrecoverable.