I do believe it's possible, yes. Whether or not Arenanet is able to do it with GW2 remains to be seen. I can't really know for sure until I've tried it out myself. But the thing is though, I think there is a decent chance that they can pull it off like they say, since they are trying to design the game to be like that already from lvl 1.
If I didn't believe it was possible, then I wouldn't be looking forward to trying out GW2 as much as I do. I'm terrible bored of wow atm, not because wow is a bad game, but because there are some serious limitations to the vertical progression model that they use. That model forces me have to wait around ever so often for the really fun stuff, like new content, new raids. If I wanted more of that kind of playing, I'm sure there are plenty of other nice MMOs out there that I could play for 6-12 months or so before I had exhausted all the content.
Wow's weakness as I see it - as well as other MMOs that follows the same vertical progression model - is that when you have gone from lvl 1 to max lvl, when you have done those 2-3000 or so quests, you are basically 'locked out' of that content forever on that character. No amount of kicking and screaming can change that. What's done, is done. That part of the game is over for that character.
So then the developers have only one thing to do to keep you busy and happy, add more content and add it often. But there's the really bad part, the developers are never able to make new content fast enough for people. The players will always consume the content they are given, faster than the developers can create more. Unless ofcourse, they fall into another trap, and make the content grindy and repetative for the sake of slowing peoples progression down until they can push out more content.
The reason I believe that Arenanet has a chance to make a really fun game at all levles - so much fun in fact that you might even forget about your leveling at times - is that they are building the game around this idea of players having awesome fun without grinding already from lvl 1, and when you hit max lvl, you are not 'locked out' of the content. The content is still there for you to enjoy for months and years to come if you so wish.
The way Arenanet is designing the game, with the dynamic events that branches out to other events, makes it more or less impossible for you to experience all that the game has to offer first time around. It just isn't possible for you to go through it all in one go, because you're not the only one that gets to decide what happens in the gameworld.
There are thousands of other players around you that also has an effect on what's gonna happen next in the world. Will they fail at an event and start of another chain of events this way, or will they succeed at an event and start another chain that way? Will the game go into night time mode when you happen to be online, and start different events because of that? Will some lucky player almost by accident pick up that rock out in the gameworld while exploring, starting a new chain of events that could change the whole map whithin the hour?
So you will not even be remotely close to finished with the content when you hit max lvl. They are making a game with a shitload of replayability compared to many other MMOs out there. Arenanet isn't 'locking you out' of the content at all, instead they actually scale you down so you can still enjoy the content you missed more or less the way it was supposed to be experienced. Not by one-shotting stuff as you do in many other MMOs.
In wotlk our guild was farming ICC for the better part of a year. Every week, again and again and again. I was so bored at times I just wanted to rip my hair out, but there was little or nothing else to do for that year. But I still loved my characters, and liked to play them, so I just had to suck it up. Now if blizzard had gone and done some of the things that Arenanet is doing - with the downscaling and the karma/token/transmutation stones - then you can bet your ass our guild wouldn't have spent close to a year farming ICC.
We would have gone back to BC and done Kara, SSC, TK, BT and what have you from time to time. And we would have had closer to the same difficulty that we had before, and still get loot (or buy loot) we could use. Instead of going back and do those raids, being totally overpowered and get loot that even the vendors barely want anymore.
This is some of the reasons why I think Arenanet has a chance to make a game that is going to be so fun and engaging for such a long time, that hitting level cap will for many just feel like a little sidenote.
How's this for idealism mate?
TL;DR: Arenanet isn't gonna make all the fun content useless when you hit max lvl like so many other MMOs do, so it has a decent chance of being fun and engaging for a very very long time.