The Diablo Immortal announcement was essentially the keynote of an already underwhelming Blizzcon this year. The game itself is just a re-skinning of an existing Chinese market ARPG with Blizzard's IP - Blizzard a company that has been known for axing titles for not meeting its quality standards. The game will be developed by Netease with one Chinese fan quoting "If EA is like a 2 out of 10, Netease is -2859".
Of course Blizzard, Diablo, and ARPG mobile games, have a long history so there is more context. For one Blizzcon is something this crowd could compare to WWDC and similar conventions. Tickets start at around $200, with travel, food, lodging, and other expenses pushing the cost of attendance to over $1000 for many people. Keep in mind that unlike WWDC or other conventions your employer will likely not pay for you to go - so its straight out of pocket and people expect to get what they pay for. In addition Blizzard has been pushing "Virtual Tickets" for $50, where most gaming conventions stream for free to users on Twitch or Youtube.
It's been over 6 years since Diablo 3, and there was a 12 year wait for that game after Diablo 2. Naturally seeing Diablo as the topic of the keynote, many Diablo fans hoped and prayed it was the Diablo 4 announcement.
Finally ARPG mobile games are massive in the Chinese market, with fremium and micro-transaction models that would make candy crush and Pokemon Go blush. With a blatant re-skin it was clearly designed for a starkly different marketing segment then would be present at the keynote at Blizzcon. To force your developers to pitch that game to that crowd is sheer incompetence, yielding what will surly be timeless internet memes such as "you all have phones right" [1], and "this is an April Fools joke right" [2].
[1]: Response to the question if they'd be porting long-requested features present in the mobile game back to the Diablo 3 PC version.
[2]: Question asked directly on stage by a brave fan.