"Answer: Market forces.
Basically, what I predict will happen is that part of every publisher agreement will be that digital distributors must support ALL their customers regardless of where it was purchased. "
I was set to rubbish this idea but on reflection that would actually work. I bemoan the fact I can't get certain games on Steam, as an example, but eg Impulse does have them. When Steam and Impulse have different rosters and DRM (call it what you will) schemes I'm going to have a preferred supplier - the one that makes my life easiest. If a major digital distributor boycotts a game, the way the bricks and mortar suppliers have done (eg the DOW2 kerfuffle), but the rest are carrying it with a ubiquitous DRM policy then I'm not going to feel bad downloading from one of them if the experience is generally uniform.
Getting that level of agreement is going to be tough though. I thought this GOO thing was a wind-up but this post actually makes a lot of sense.