The issue isn't with the game, nobody cares that Blizzard are releasing a mobile game that makes sense.
The issue is the clear disconnect with the fact they thought it was a good idea to announce it at Blizzcon. It was a move that was so absurdly stupid that 4 years later I still can't believe it happened.
To hype up one of the franchies your best known for to your majority-PC devoted playerbase, who have spend possibly thousands to attend your event all to announce a mobile game. I can't even comprehend what leads one to this conclusion. It shows such a deep lack of understanding and disconnect from their community that it creates concern that they have any clue at all what they are doing. That one single act makes me think D4 is going to be bad, because to make such a decision shows me they just don't understand their community anymore.