Consider what it would take to replace DNS with a distributed system. In a global namespace, names have value. You can't just operate on first-come first-served - there needs to be a system that ensures that names go to whoever wants it most. In other words, the challenge to be solved there isn't the technical one of having a DHT - that's a mostly solved problem - it's how the hell to make the names cost money, and who the money goes to. (And if you don't think the names should cost money, how do you propose allocation should work?)