Ok, so if the issuer just disappers with your fiat, you can do nothing about it, right?
Well, the centralized unregulated exchange (which is most of them right now) can always disappear too.
The difference is, when you have few centralized exchanges, they are easy to shut down.
When you have decentralized individuals, nobody can shut them down, except they can disappear on their own, just like centralized unregulated exchanges can do the same when they think they got enough and it's time to go to a beach
but then I am sure there has to be some sort of reputation system in place, that allows to record feedback about a certain participant, like on ebay for example.