The no-monthly fee thing is the future of online massively multiplayer games. Make no mistake, just because Guild Wars is completely instanced, does not mean the model will not eventually consume the MMO market as a whole. While the game in execution may not be perfect, the financial model is in my opinion, the forefront of the future.
Guild Wars will only fail once the community loses interest. My interest faltered a few months after release. Mind you, I was a rather long running alpha tester, so I had been exposed to it for quite a while before that. The main problem I had with it is that it was simply too much of a time investment. Leveling up may not take a while, but organizing a team of eight often took longer than playing an actual match. I could not justify that kind of time investment. How long will the rest of the community remain interested? Given that no monthly fee is required, which means people can come and go as they please, I imagine it will last a very, very long time.