The key difference between BTC and fiat is not who does not control it. That is the key advertised difference. The key difference is that fiat is issued for credit, and BTC is issued for work already done (hashing to prove transactions). Long, long ago, homo economicus doped out that issuing money against future production enabled that production by providing working capital for its producer. That enabled the risk of production to be decoupled from the skill of production — specialization of financier and entrepreneur — the synergy that makes a market economy go.

BTC proposes to ignore financing of future activity. Banks will not be able to create BTC the way they can create fiat. Yes, banks can create a coin derivative of BTC, much as they created checking accounts against fractional gold reserves, but in those days gold was pretty much a stablecoin, whereas BTC is intentionally deflationary, which means that no one will want to borrow it at a positive interest rate. Only an infinitely expandable stablecoin can serve as the basis for industrial finance, and, when the smoke clears, the medium of exchange must also be that unit of account.

All of these problem make the security problem moot, because, even if it could be solved, BTC would still be pre-historic money in a shiny new wrapper.