1. All the best domains are taken.
2. Namecoin was never meant to be a currency.
3. Plugins and having to change your DNS have never worked in the past, why now?
4. Merged mining
For all of the above reason. NMC will fail.
1. The current dns-system will probably fail then too, because
the best domains are taken.
2. It doesn't fail at being a decentralized-dns-system by being a bad currency, because it was never meant to be one.
3. Maybe because of its censorship-resistance.
4. Again, it was never meant to be a currency, don't treat it like one and merged mining becomes a plus, because you don't even need to leave the bitcoin-network to join.
For all of the above reasons, you fail.