I think the key lesson from history is that there are a number of factors that have to align for a paradigm shift to take place, but that from time to time they do happen.
There are tons of examples from history of things that people thought would never change or technologies that were thought to be dead in the water that ended up completely displacing previous alternatives.
Pretty much every technological communication technology falls into that category.
Adoption will be incredibly slow or won't happen at all up until a certain point, where the tail of exponential growth kicks in and suddenly, it's everywhere. Look at mobile phones: it took years after the introduction of mobile phones for them to become ubiquitous. I doubt anyone could have predicted that there would be more mobile phones than people in the world within ten years.
I'm not saying that namecoin will take over. I do think it is possible, depending on what happens with the regular DNS system. At the moment, the need for an alternative is just not there for most people. That may not be true forever.
Look at IPv6: from the current state of IPv6 adoption, it would be possible to throw up your hands and declare it a failure, and say that it will never happen. I think that there will come a point where the price of IPv4 addresses is high enough to push widespread adoption, and at that point, everyone will be using it at once. And afterwards, people will look back and wonder why it took so long and how things could change over night. That's just the way things work.