There's still a little oil left to be thrown onto the burning inferno of Diablo Immortal hot takes. According to Kotaku's sources - "two people familiar with Blizzard's plans" - there were plans to quietly announce Diablo 4 at the show, but the studio opted not to as there wasn't much to show so soon into production. For whatever reason, Blizzard decided to run purely with the NetEase co-developed mobile spinoff Diablo Immortal, which I think looks decent, but judging by the internet's general response is now apparently the worst thing since sliced plague.
Update: Kotaku have clarified that while a video announcing a new Diablo video was produced, Blizzard deny that they planned to show it at BlizzCon.
Per Kotaku, Diablo 4 is still very early in development. No CG trailer, no gameplay footage. If they had talked about it at the show, it would have just been a brief side-mention after the Immortal announcement. Sources say Allen Adham - Blizzard co-founder and possible Marvel superhero, given that name - was going to appear in a video explaining that Diablo 4 was in the works, but they still had a ways to go before it was show-ready. Admittedly that would have been a pretty underwhelming statement to end the Diablo keynote on, but in hindsight, it may have helped.
It sounds like it would have largely been a reiteration of Blizzard's pre-BlizzCon statement on their official page here. Stating that there's more Diablo projects in the works but they would be shown "when the time was right". Unfortunately, some took that to mean that the sequel would be making a showing sooner rather than later. It all feels like a comedy of errors at this point - assumptions, presumptions and confusion over expectations. For all the fire and brimstone raining down right now, it was all sparked by what feels like a relatively minor PR fumble.
Ah well, at least we can agree that Diablo 3 on the Switch looks pretty nice, right? Seriously though, folks need to chill. There will be more skeletons to click on (with a mouse, as the dark lord intended) eventually.