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There has been no news about Diablo for years, the last "content" update for DIII was over a year ago, and the one before that was 3/4 years ago. Its a game that has divided fans, but still the majority of people like it.
Blizzard started leaking info that there would be Diablo news at Blizzcon (which is a $100 ticket, before you factor in travel/accommodation). People understandably got excited, and Blizzard tried to dampen down speculation (after they'd sold out their tickets ...), but didn't explicitly say what to expect.
The Diablo announcement was made at the end of Blizzcon, typically the most prestigious slot. They tried to imply that this is in addition to any team that might or might not be working on DIV, but that didn't go down well, and the fact that the company they teamed up with are a well know Chinese company who's bread-and-butter was making mobile Diablo/Warcraft clones lousy with microtransactions made people understandably disappointed.
The TL:DR is that it was a weak announcement that was made much much worse by miss-management of the hype leading up to it by Blizzard and the choice of when to announce it at the Con. People over-reacted but Blizzard should have done better and not given them that chance (IMO).
Can someone ELI5 what the Diablo ruckus was this week? It's a PC game with a committed PC fanbase and they wanted to make the next one mobile only? That about it?