I do not understand why in the basis of a decentralized system, you are going to add a centralized registrar?
I should have made it clear: I imagine there will be an arbitrary number of registrars. They will compete to provide the best service (fastest updates of the DNS database, lowest prices, etc).
If you were willing to do the proof-of-work and insert your own updates into the bitcoin block chain then you could be your own registrar (I assume most people won't be willing to setup the necessary software, run it, etc. just to register a couple of domain names).