The technical considerations are uninteresting here: all of Blizzard's titles are optionally built to use OpenGL, letting them avoid the golden handcuffs of DirectX & Friends. So if porting is cost-effective, why would they do it when virtually all of their revenue comes from Windows systems?

I say: gatekeeper-free subscription gaming appliances.

There's no evidence the total Windows PC experience is getting any better or cheaper for the uninitiated, there's no evidence that Playstation or Microsoft will bend over and allow third party platforms on their consoles, and all of the new-era Desktop App Store UXes force significant technical and billing restrictions that would eat at Activision Blizzard's subscription and in-app business model and reduce their control over the game experience.

And I don't think this is a bet that desktop Linux/x86 is going to be relevant for gaming. It has no inherent advantages over classical Windows.

So I think this is a gamble that new-era quasi-consoles based on Android+Linux/ARM+x86 are going to succeed to some degree (be it the Ouya or Steam Big Picture or some Samsung Generico-Colossus that can spit out WiDi or whatever).

A keyboard, mouse, and controller capable Linux platform with no overarching gatekeeper and adequate gaming horsepower is going to be extraordinarily cheap in the near future. If Blizzard Activision gets their ducks in a row now, they'll be ready to jump into the first one that offers ten million users, a reasonable hardware target, and a bullshit-free content delivery mechanism.