> The entire concept of fiat currency is abstraction

Then why does the US still maintain over 8000 tons of gold at Fort Knox and elsewhere, 45 years after closing the gold window?

Of course even before 1971 paper currency was an abstraction as well - it was a piece of paper to get a commodity, not the commodity itself.

An important part of the abstraction in any fiat currency with a modern central banking system is that the quantity of future financial obligations in that currency exceeds the broad quantity of money (because the greater part of the money supply created on banks' balance sheets literally is debt, and the government insists your future taxes are paid in that currency too)

Bitcoin isn't dismissed as "fairy money" because it's digital but because it doesn't have a similar rather large built-in reason to assume there will be a non-trivial demand for it at a particular point in future.

However [purposely] opaque certain other classes of financial asset are, they tend share the general property - which BTC doesn't have - of representing one entity having a future claim on another's resources.