I simply don't get how can you use a DEX for FIAT<>Bitcoin? I mean there are no visible smart contract on the FIAT side.
it technically can't be done. that is, the process cannot be made
fully decentralized and trustless. outside of face-to-face cash transactions, fiat always entails third party custody. you can distribute that third party custody beyond a single point of failure, but you cannot fully remove the need for third party custody and trust.
bisq.network is a good example. it's a huge improvement over centralized exchanges in that it uses a DAO to handle escrow/arbitration/voting. but at the end of the day,
this is a pretty accurate description:it never has custody of your funds
It does. There is a security deposit and escrow. It might be temporarily owned/held by more than one individual, with a complex way of keeping it away from any one person or company, but the money isn't just peer-to-peer. There is a central system that it has to go through and be held by.
It technically could be done, if any governments were to switch over to a decentralized and open sourced cryptocurrency. However, I don't see that happening any time soon, if ever, since they like the ability to print currency.
I could technically see a crypto built whereby say something similar to DASH exists. Basically you have an entity entitled to some amount of the appropriations year over year. That could be the government. The rest of the rewards would be used to pay off miners securing the network.
This, of course, gives them less control over the money supply, which they probably view as quite bad and needless is the overall problem.
But if any auxiliary country decides to try doing this as a novel feature, it could be a part/function of reality of a DEX.
Until then, yes, it's purely fantasy and relies on some sense of decentralization in the best case scenarios.