technically you're correct ... but spending power = spending power , any way you define it ... it's simply a matter of liquidity when it comes straight down to it ... if i have 1/2 of a billion dollars in fiat currencies , and i've got real estate , stocks , bonds , airplanes , collector cars , and gold bars worth 1/2 of a billion dollars ... i have 1 billion dollars worth of spending power aka wealth ... i'd just need to hock the real estate , stocks , bonds , airplanes , collector cars , and gold bars in exchange for liquid capital if i wanted my wealth measured purely in fiat terms ...
See, the point is, both fiat AND bitcoin are a part of your wealth. You can sell your gold bars, house, and whatever else for bitcoin as well. So that's why what you said is a fallacy. You can only compare fiat and bitcoin, bringing wealth into the picture doesn't work.
Even if bitcoin replaced all fiat, there will still be people with houses, with cars, with planes, with patents, with stocks. And they can sell those for bitcoins. Personally I'd rather have a mansion than bitcoins.