Seriously... I don't mind having your own personal opinions but If you HAVE to drag in some other games to compare against Blizzard games don't compare the sub quality trash like TL2 and most other MMORPG's.
Whether you like these Blizzard games or not, no other games last longer in playing time by THEIR fans.
TL2 hein ? compares to D3 like an ant to an elephant in playing numbers and playing time and for the moment I don't even like the non trading D3, but that doesn't mean I can ignore simple facts D3 is a gigantic commercial success and is played by zillion of players who have fun.
I am no longer playing HS and I don't like SC2, but when I enter Azeroth I can feel the difference with other MMO's, from the first critter I encounter to the very last Dragon I see.
All of their games are simply played massively whether you "like" them or not.
Of course Blizzard games have limits. They appeal to many players but not to ALL. They are limited to address some typical kind of players: those who invest a ridiculous amount of time to just play ONE game and the typical hardcore dudes. Blizzard's main public is the "I have 8 maxed out avatars in Diablo 3 spread over PC, PS4 and XBox360."
Blizzard failed to address the REAL casual players on new media btw, like the Farmvilles of our time. That's a market still right open to them. Even their first iPad game is simply too harsh to conquer that very lucrative casual iPad market in numbers like Supercell does.
At the same time, Blizzard is working and developping way over the heads of their public even. The vast majority of players have NO CLUE about game design. The transition from a single realm play to seamless cross server play is way too difficult to grasp for the average idiot smashing a key or a joystick.
So yep, the OP is right: they are mostly too good to be limited to an industry that addresses a general IQ of 95 and has the patience of a 15 year old kid with the highest form of ADHD and is color blind.