I think the term successful is also up for debate. One might consider a game that DIDNT' go free to play yet (FFXIV) as being successful, but in comparison to subbers, the numbers are so low some people couldn't honestly say having 2 mil players is successful against a game with 7 million players.
I think GW2 was successful for what it wants. I think that the players they currently have are loyal and I think that ANET is okay with that. They aren't looking for more players though imo which does give me a feel that they've prolly given up that path. Kind of like a "this is as good as it gets".
Yes, they're putting an expac out, but at the same time for the LONGEST time everyone was under the impression there would never be an expac, simply more living worlds or the new things would just be patched in over time. The expac can be seen either as the game is successful or as an omen that they need more money.
Do I consider GW2 successful? Overall, meh, maybe? Personally, sure. I play it once every few months, maybe less but I enjoy the short time I play, and since it's free I don't really have to worry about a sub.
Would I pay for GW2 if they had a sub? Ohhh no. Not at all for me. The game for me is a casual event grinder. Sure I pay for dyes or weapon skins. I've thrown a couple of tens down at them. But I could never pay monthly to play, I simply don't enjoy it -that- much.
I think that if most games were looked at it in that view "would I pay 15$ a month to play this", it'd be a lot easier to see which ones where successful or not. Some get played simply because they're a one time fee and that's done. Others have tried a sub and failed, few have tried the sub and succeed.
All of the above is personal opinion. I'm not a video game economist or a business major for video games. I base success simply on how I see things. Kinda like with destiny. A billion people complain about the game, how they hate it, yadda yaddda, a billion people play that game. Sorry yo, that games successful. It keeps people, lots of people. Not all so casual one time logs in a month people either. Retainability even through the hardest times.
Oh, and FFXIV is doing just fine for itself. Made it to an expansion without going free to play and still boasting healthy numbers. Props to that.