One thing I've never understood is the attitude that just because you like something it's fine to force others to use it. I like chocolate icecream, drive a Toyota and use Firefox but I'd never dream of telling people they had to use them. Yet for some reason Steam users often seem to be… affronted, in an almost religious sense, when people don't want to be forced to use Steam. Which is, often, and amusingly, coupled with outrage when they themselves are 'forced' to use something they don't like.
Steam got where it is by bundling, from Half Life 2 (x million copies sold = x million baseline users of Steam) on. I'd be surprised if a single person on this forum actually downloaded Steam 'voluntarily' rather than got it installed via bundling. From Valve's view the entire point of Steamworks is to drive installs, they aren't doing the loss leading for it out of the goodness of their heart but because they know they'll make money from it in the longer term from either DLC or defaultism.
Closed system, MS has no need to allow Valve access. Allow publishers their own space on Live Marketplace and make money the same way they do for physical copies- ie licencing fees- if they really want to stick the knife in.
In that scenario MS would have a hardware specification for the nextbox which would be, essentially, a closed box PC running AMD/nVid video on AMD/ Intel chips, develop for the nextbox and you would be developing for PC simultaneously. Since most publishers don't fundamentally 'like' PC and far prefer making or saving money to taking principled stands they'd make a unified SKU and rely on AMD/ nVid (or 'NextBox compatible' labelling) to provide compatibility.