I agree throwing a temper tantrum is dumb, and people who go on twitter and yell at developers and otherwise overreact are silly. However it's true that Blizzard really misjudged the audience on this one. The reason I find some common ground with the complainers is only in the fact that none of the current Diablo fanbase really wanted this. It's a sign Blizzard wants to expand the Diablo brand. But, is that really a good choice? Personally I'm not sure it is. I have a hard time imagining the world of Diablo ever being as popular as Warcraft, since Diablo was always heavily steeped in European/Christian symbolism. More generally it's moodier, darker aesthetic will be more niche than the bright one of Warcraft.
By the way, given this is obviously targeted at Chinese audiences and is made by Chinese devs, we can expect: no skeletons, no excessive gore, no religious symbolism. Also "family friendly", lol.
There's speculation that another project was cancelled, as Diabloimmortal.com was only registered in October, Blizzard recently put out job listings for high level employees on the Diablo project, and the Immortal developers didn't have many responses for what the game would be like. It all seems to suggest that something was cancelled, either from Blizzcon or in general.
In any case, what's happening to video games is what's happened to big name movies. Have you noticed how boring and generally low level big budget movies are in the writing now? It's because they don't do things like write jokes which are hard to translate. They write jokes and everything to be understood by as general a human being as they can, so humor stays at a far more basic level.
We're seeing the exact same thing with video games, now. Diablo did have some appeal in how it boldly used religious symbolism. Pentagrams, magic circles, all sorts of occult imagery, plus the cathedral layout of Diablo's lair, it uses European religious iconography heavily. That was all cut from Diablo 3. Then with Chinese censors, games like Rainbow 6: Siege are removing skulls and skeletons, and we can expect the same moving forward from Blizzard games (previously, they would make two versions of the game with different assets, like in WoW, but that's becoming less common). As they move games to be as palatable to the largest audience possible, everything becomes so dull and boring it loses appeal.
Not to mention NetEase is apparently renown in China for having horribly P2W mechanics in their mobile games, so.