There's no guarantee that $ would still exist 200 years from now or that it would still be the global currency …
It’s about buying power. You have to use USD for this comparison NOW because it has consistent buying power. Bitcoin doesn’t. Usd will only be replaced buy something similarly consistent. If you get your paycheck and the next week you have half the buying power (like with btc) nobody will adopt that.
What if each block contains 100000 transactions instead of the current 2800, little infrastructure change is probably needed to change that (if any other than code and signalling). Each of those could pay $1 and overpay on the current system.
The is a problem with that already and you know it (Blocksize). Bitcoin isn’t made to last forever, and I am just now thinking about the future.