I'm a Diablo fan, a massive one in fact. I didn't pay to go to Blizzcon (someday...I hope), but I did sit through the entire opening ceremony waiting for a Diablo announcement. Although I very much enjoy most of Blizzard's other IPs, this year was the year for Diablo. They announced the mobile game, I was kinda disappointed, and that was that. After I watched some demo gameplay I became interested. I didn't immediately fly into an outrage. Many others didn't.
Most rational fans know the next main game is coming. It's okay to be angry, and not like this announcement. It's okay to want confirmation officially of D4's existence. The line begins to blur when people declare the franchise is dead, that Blizzard killed a generation of fans, they spit on the fanbase, or that we need to create and sign petitions to get the game cancelled. At that point the way the anger and channeled and the message is delivered takes it a few too many steps and it becomes childish, embarrassing, and unhelpful.
I'm mostly sad now that it's currently difficult to go anywhere Diablo or gaming related without seeing a (at times) sea of bile and aggression. The Diablo subreddit is more unreadable than usual. The official forums are (amazingly) even more negative and hyperbolic. I love this franchise, and love to discuss it. Pretty hard to do that right now LOL.
Oh well. This will all blow over eventually. I'm personally really excited for Immortal, and for further announcements. At least Blizzard are giving Diablo some attention between (well spaced) main entries.