Why not? By your definition, Guild Wars has a single player component and by being always online is bad design.
Apples are red. Therefore anything that is red is an apple?
I have never played Guild wars, so I don't know what the game play is like. But from what I have read and heard about the game, it is a non-typical MMO in the form of instance game worlds (I could be totally wrong about this). The point is, it is designed mainly to be played both with and against other players. Hence the name "Guild Wars".
Diablo 3 is a very different game. The game play is very VERY similar to games like Diablo 2, Titan Quest, Neverwinter Nights 2 and Torchlight. The main focus of the game isn't PVP fighting other players. It isn't only "Possible" to play the game solo, it is actually designed with single player game style of play as part of the base design. Maybe not "The Main Focus" but absolutely as part of the base design.
Now, if I am wrong? If You believe that an average player could play Guild Wars solo and complete every single mission. Could defeat the end boss solo. Could fully enjoy and realize 90% or more of the content of the game without encountering a single other player, then YES. I would say Guild Wars has a single player component. But I don't think that is the way it is designed.
DIABLO 3 IS!!! Pure and simple, I have never once encountered another player in the 30-40 hours I have put in thus far. I have defeated every Boss I have encountered without major issues or problems. I have been able to complete every single quest put forth. I have encountered nothing what so ever that even remotely made me wish I had another player to tackle in game. And I am not an Elite player.
If you can't see that simple truth, then I can't help you.
Diablo 3 is very like other games that have an off line component. Guild wars is not (to my knowledge). The ONLY thing that ties the two together is the a single design decision (that of being online) that, if removed (which is to say made playable as either an online OR offline game), would not fundamentally change anything about the game play. Can the same be said of Guild Wars?
It would be analogous to using a tablet as your cell phone. yes, you can do it, but a cell phone is designed to be a cell phone (i.e. MMOs are designed to be Massive and online). A tablet "Can do online calling", but that isn't all they are made for. and they aren't as good at it. hence it could be seen as a "Tablets are a Bad design
FOR A CELL PHONE". but that doesn't mean that Cell phones are bad designs.
If I am wrong about this, prove that Diablo 3 would change fundamentally (for the worst. Clearly it would change for the better) if there was an option to play off line (single player or Multi-player in a LAN environment).
At the end of the day, people like you cling to the single lame "But Guild Wars" claim because it is the only instance even remotely that supports your claim. yet to do that you have to ignore all of the other games that disprove your claim. One outlier (always assuming that Guild Wars is) does not make an argument. Particularly when I can provide 4 solid other games that counter the example.