The new Diablo game - why it's not an expansion

Hello boys and girl. My first actual post that isn't about Star Wars: The Old Republic. Wooohooo. Time for a celebration. Cheese for everyone.
Okay, so BlizzCon is just around the corner and for once we don't have to speculate about the WoW expansion. However blizzard is recruiting for a new diablo game.
It is most likely a new expansion, but let us look at why that may not be true. You know, for fun.
We all but exhausted the options for the diablo universe. We killed the fusion of all the seven evils and even murdered death itself. There is not much else to do for the Nephalem.
But this is still a small problem. I have a pretty good idea for what could happen in a potential expansion. The big problem is that the game is moving away from campaign game.
Everyone is doing adventure mode these days. Adventure mode even got a new feature, Kanai's cube, and a whole new zone. The zone is filled with new monster types and has new info from Guru Pathik. Err I mean Abd al-Hazir. Oh, it is also technically the sequel of the campaign as Zoltan Kulle is back and even mentions that he brought back himself again. And all for free, Is there even a point in making new expansions?
So yes. It is not that hard to understand why Blizz would rather make a spinoff. Perhaps one better fir for E-Sports.