You can't have decentralized DNS without some form of shared state, where the shared state must resolve the problem of "if I use key X to change the owner to Y,

You can't have decentralized DNS without some form of shared state, where the shared state must resolve the problem of "if I use key X to change the owner to Y, but then later on someone hacks my key and sends a message to change the owner to Z, how does the world know that Y came first?". This is equivalent to the double spending problem, and so a solution to this problem implies blockchains or some other architecture that can support a cryptocurrency.