If the productive capacity of the economy declines your capital will be inflated away. Money is a social construct built on a stable or growing economy.
The FED can target either interest rates or the money supply. It could very well adjust supply to meet a shrinking population pool. Otherwise post war losses of many able bodied men would inflate away economies.
There could be. Our example hasn't considered productivity gains due to capital improvements or tech advancements. We may not need the same population to produce the samd product count in the future.
