I agree.
Money that competes for blockspace is mostly denominated in fiat:
  • Miner costs are denominated in fiat
  • Opportunity cost of using bitcoin is denominated in fiat.
Opportunity cost of using bitcoin is a function of bitcoin competition with meatspace entities.
For example:
  • If it costs me 1$ to send 10000$ of remittance with bitcoin I will use bitcoin.
  • If it costs me 1000$ to send 10000$ of remittance with bitcoin I will use western union.
If most economic decisions of "should I used bitcoin or $SOME_MEATSPACE_COMPETING_PRODUCT" are denominated in fiat, the competition for blockspace will be a function of fiat-denominated value (and not bitcoin-denominated value).
There are cases in which bitcoin have no meatspace competition (like censorship-resistant money) and those cases might cause a decoupling. But arguably most competition for blockspace at the moment are driven by competition-with-meatspace-entities.