It's pretty much irrelevant to compare the longevity of Guild Wars 2 to games like WoW and even Guild Wars 1 because there has been a structural change in the past decade of how we play games. It seems like each MMO after WoW has 'lived' for less and less (excluding those with major issues, eg. AoC). This is the same for all games.
While I played The Sims original for hours of hundreds of hours, I barely clocked a hundred with The Sims 2. The Sims 3 lasted me a weekend. Similarly, I played Morrowind for hundreds (if not thousands) of hours over several years. I sunk a good hundred or so into Oblivion. Skyrim lasted 40.
With all the options out there it has become very easy to jump from game to game. The most relevant example would be downloading the latest 'game of the week' on iPhone/Android. In my experience, I tire from games very easily now. Maybe this has something to do with getting older or perhaps the internet has made us crave instant gratification and change.
Now I'm still enjoying Guild Wars 2 and expect to do so for years to come. My guild is also not having activity issues. However I don't believe we will ever see MMO's 'last' as long as they used too. The ultimate test for this will be Blizzard's Titan. However I expect that to share the same fate.
I'm completely sick of hearing about the Mystic Forge. It's what this game revolves around. Want an interesting item? Try the Mystic Forge! Something wrong with the economy? Mystic Forge to the rescue! Something else screwed up with the game? Have no fear, there will be a new recipe with the next update... for the Mystic Forge!
Grind, grind, grind for the Mystic Forge. Toss your stuff into the Mystic Forge! All of it! Don't hold back, the Mystic Forge must be fed. Bow down to our new lord, the Mystic Forge.
What, they couldn't think of another way to introduce interesting items to the game? And while I'm at it, what the hell is so legendary about weapons created by dragging crap from all over the place into four slots? What's the legend behind those weapons? Who were the heroes that wielded them in days gone by? What fearsome creatures did they slay with them?
This damn game has no heart, no soul. It has is a number of uninteresting formulas, and hordes of people clogging up the isle in LA's Black Lion office, spending their days trying to get ridiculous amounts of lodestones or whateverthehell else the damn forge want them to feed it. It's pathetic, and I can't believe ANet is responsible for the abomination.
What kind of drugs are you on, and where can I get some?
Still having a lot of fun.
Population has definitely spread out but I haven't noticed an overall decline.
Will continue to play. Have plenty of armor/weapon dungeon gear in mind, one more alt and I want to get one legendary weapon. So I'll be playing for a while.