Well fuck if you didn't completely miss the point.
Lets go with your analogy shall we?
The Diablo Immortal reveal is a 4 year old throwing a tantrum because their daddy gave them banana flavoured icecream instead of a Banoffee Pie with toffee sauce. They advertised what was initially a MOBILE GAME to an audience that was majorly PC focused with likely a few console players thrown in using a franchise that had at that time been on PC for twenty two years and console for 6 years.
Blizzard announcing a mobile Diablo game in front of what was primarily a PC audience was like if Ferrari announced that their brand new vehicle was going to be a unicycle.
The game could have turned out amazing and it wouldn't matter, you don't announce a mobile app as a "big thing" to an audience of people who have been playing your games on actual gaming systems for years, and who paid money to go to your convention. If it had been announced seperate from Blizzcon, or announced in a way that made it clear that it was a side project rather than making it out to be something big for the Diablo franchise there would have been FAR less of a negative reaction.