Wyatt literally said yesterday on Rhykker's stream that they aren't abandoning, nor have they forgotten their PC playerbase. They've got multiple projects in the works, but Immortal is what was ready to show, so they're showing it.
So I dunno, how much of a stretch is it to imagine that they've got a mainstream Diablo game in the works based on Wyatt's comments?
I'm not saying it's not happening, I think most people understand it's most likely happening, but the fact is they didn't announce it officially. Bethesda threw together a 30secs teaser trailer showing some mountains to announce that they were in fact working on TESVI even though they had jack to show for it(not even the place it'll take place in, although fans analyzed the trailer to figure that out to an extent). Game isn't even planned for 2020. But Blizzard did not do this. Maybe because they do cancel games. Starcraft: Ghost and Project Titan come to mind especially, since those were very well known too, and yet neither ended up existing in the end.
Just saying, if they did announced D4 is being worked on, with a quickly thrown together trailer just showing Diablo IV and some Diablo silouhette in the dark or whatever, after the mobile announcement, people would have been "eh, at least we got D4, mobile sucks but whatever". But they didn't do anything, and that's why they're getting crucified. Especially since the mobile game isn't even like, a port of D3 or D2 remake in a PC/mobile simultaneous release or something like that, it's an oversimplified mobile designed game with Diablo IP slapped on it. It might be fun for all we know, and not be crippled by microtransaction shit, but they also didn't bother talking about that to reassure the few people who might be interested in the mobile game either.
Sucks for Wyatt Cheng too, guy turned D3 around in my opinion from a fairly mediocre game to a pretty fun game but I think a lot of people are going to remember him now as the guy who was somehow "as bad as Jay Wilson".