This was met with huge backlash from fans and following that backlash, Blizzard has stressed during an interview with Polygon that they still had several Diablo projects in the works that have yet to be revealed.
According to Allen Adham, executive producer and Blizzard co-founder, “We have said that we have multiple Diablo teams working on multiple Diablo projects and that remains true, even after releasing [Diablo 3 for Nintendo] Switch and announcing Diablo: Immortal. We still have multiple Diablo teams working on multiple unannounced Diablo projects. Diablo is a tentpole franchise for us. And it always will be. We love it. We hope our fans understand what we’re saying when we say that.”
To be fair, Blizzard did state ahead of BlizzCon that the game(s) that many players were hoping would be announced would not be revealed, so while Diablo Immortals might not be what fans were hoping for, nowhere did Blizzard state that this doesn’t mean that Diablo 4 isn’t in the works. There have also been rumors that a Diablo animated series for Netflix could be in the works, but that announcement was absent during BlizzCon.