This is one thing that I have commented on in other threads as a future possibility: just as central banks have gold reserves, they could have reserves of Bitcoin, which in practice is an enhanced digital gold. But if it happens will take time and also I think it will take a component that they almost have, which is a high degree of control of Bitcoin by the authorities through regulations and KYC obligation.
ETFs just opened the door for mass accumulation by institutional investors. This, alongside the seizure of BTC from illicit activities (particularly by the US government), tells us that external forces will drive Bitcoin away from its intended purpose. Backing Fiat with BTC should be a no-brainer in the future. We're already seeing pro-Bitcoin politicians (Milei, Bukele, RFK Jr., etc), so anything's possible.
I'm yet to see if central banks are going to like this idea, especially when they use to "print money" to manipulate the economy (something which can't be done with Bitcoin). The future is widely unpredictable, so we can only hope for the best.