Why not use Steam? Because Gabe is going to die someday, and there's no reason to think that his successor will inherit his good judgement. Locking yourself into a monoculture isn't a good idea when your whole business can be smashed by one poor decision by the company pushing that monoculture.

I fully agree with your opinion on this. I also can envision a future where AAA games can run fine on the browser, which would act like a universal platform. Not sure if this will ever come to pass, but it is technically feasible. I just don't like javascript :)