You know, we do have a historical example to refer to regarding population declines, and it was good, not bad.
Specifically, the soaring standards of living and wages following the Black Death.
While GDP doubtless went down, per capita GDP soared as did wages for those in the bottom half of the population because of a labor shortage.
In fact, people became so well to do that they could afford linen underwear, which became linen rags, which became linen paper, and led to the first information revolution.
If, (in real terms) as a result of demographic issues, social security taxes have to rise 20%, but wages for the median person go up 40%, everyone wins but the Richie Rich types.