I get why people are pissed about the Diablo Mobile announcement, But honestly you can't say you are surprised by it. Blizzard and Activision are one in the same and they have been since around 2015 after Activision bought themselves (along with blizzard) out for Vivendi.
They are one in the same company, this move isn't surprising. Just look at how Overwatch is handled with Lootbox's and how Heathstone/HOTS is also handled.
Its par for the course that was going to happen. A new release of Diablo wasn't going to come out that they couldn't make money on daily.
Blizzard isn't some indie company that you think loves you. They are a business and there goal is to make $$$ above all else.
That isn't to say there isn't developers who do love you cause I'm sure there is. But the CEO and share holders pull the strings and profit comes before anything else.
Welcome to Games As A Service.
Edit: Also to add if and when Diablo 4 actually happens Activision-Blizzard will have some way to make money from it daily.