The obvious issue is people are tired of the current state of WoW.
Sc2 is just hanging on by a thread.
I think if Diablo 4 releases and is exceptional, it would restore my faith in Blizzard. But I'm remaining cautious. Especially after buying RDR2 and still being unable to play it almost a week after launch.
On the same note, i bought COD:MW and that game is outstanding.