Personally, I don't dislike the idea of D3 on my phone. Mobile games are a cheap thrill while your on the can, waiting at the doctors office, etc. They NEVER replace PC/console games for me.
Having said that I think the BIGGEST uproar about Diablo Immortal was it was Blizzards BIG Diablo franchise announcement for the last Blizzcon. It was just really underwhelming news when we all had such high hopes.
Imagine paying to go to a Nintendocon expecting to see some awesome new stuff and the BIG announcement was Super Mario Run 2.