I assume the reason they outsourced this game is because their own guys are working on Diablo 4 internally.
I doubt it.
A/B purchased King (Candy Crush) a while back and if you look at earnings for PC/Console each year compared to mobile, it's clear what direction the company is headed.
http://www.youtube.com/watch?v=BOVfw5O5c1g&t=2m9s"Uhm... In terms of, uh, Blizzard's approach to mobile gaming: Many of us, over the last few years, have shifted from playing primarily desktop to playing many hours on mobile and
we have many of our best developers now working on new mobile titles across all of our IP's. Some of them are, uh, are with external partners, like, uh, Diablo: Immortal, uh, many of them are being developed internally only, uh, and we'll have, uh, information to share on those in the future. I will say, also, that we have more products in development today, at Blizzard, than we've ever had in our history and our future is very bright."
They do go on to say for the "diehard PC fans," they have "multiple Diablo teams working on multiple, unannounced, Diablo projects" and they "haven't forgotten about [us];" however, even if they are working on Diablo 4, I have lost all faith in their ability to produce quality games after Diablo 3. Considering they thought headlining a mobile game to an audience of PC gamers was a good idea, without any other teaser or announcement, shows how out of touch they are.