Precisely this. It's not that feeble because you must remember what Blizzcon is. Know your audience. Blizzcon audience has paid $199 to get in. They are quite hard core PC gamers. If your main show for them is something they are not interested of course they are gonna be unhappy.
If Blizzard wants to change Blizzcon they should tell the audience that "Blizzcon 2018 is focused on mobile" before they buy the tickets.
Don't suddenly change the theme and expect a good reaction
Well, absolute outrage is not absolute. This isn't something that's accurately metered. As with everything in outrage culture, the media tells us the outrage is palpable and we simply accept it. Users voicing their displeasure on reddit / twitter / youtube in any capacity is all it takes for the rags to twist this into an apocalyptic, mouth-frothing rage machine. It's a complete joke. The barrier to typing a few words for the world to see is paper thin. Displeasure isn't rage.
It's easy to manipulate discourse this way because anything can be construed as utter outrage.
What is unique in gaming is the feedback loop that happens. First the media responds by saying the guy is an asshole. Then the gamers respond with more vitriol. Then there is "Everyone hating the mobile diablo is a misogynist" kind of tweets. Rinse and repeat.
Do note I'm not justifying it. Merely trying to reason about why especially in gaming these outrages escalate into insane heights instead of dying in meh after a day or two.
We have been seeing this kind of vicious circle elsewhere too lately. As an example the latest Star Wars film where it ended up so bad that if you didn't like the film you were a Russian troll.
The complaint here is that Blizzard is abandoning the people who made them rich, and the off the cuff remark about phones is proof of the latent disrespect they have for their customers. The fact that it was an awkward unscripted remark is what makes it sincere, and shows what lies just under the surface when they don't have their marketing staff to edit their press releases.
"OH, they're doing it to make more money" isn't a good excuse. They already make enough money. Blizzard should reciprocate loyalty to the people who are loyal to them, and that breach of respect is why people are mad.