this crossed my mind a few weeks ago...
if they:
- licensed Direct X from microsoft (I see this being the hardest bit)
- made friends with EA/Origin , UPLAY etc etc (there would always be the odd game that wasn't on steam!)
- GPU manufacturers put as much effort into driver performance as they do for windows versions
- either a built in media player + web browser + voip and so on, or compatible with open source ones such as MPC-HC, firefox etc (which to be fair would almost be a given, as would probably be linux derivitive!)
then it could maybe take off. would still questions around virus vunerabilities and so on.
personally, I think a more pragmatic approach would be for Gabe and other game industry giants to pressure M$ towards a cut down, gamer orientated release of windows (hell if it was M$ doing it, they could maybe even make it able to play xbox games, which in my mind would give a fair kick to the sony camp!)
also, while it may seem fairly simple from a consumer point, security (https, SSL etc.)is a big part of the work that goes into maintaining windows, so would need some heavy assurance before making secure payments over it and so on!
for me, if there WAS such an OS, I wouldn't replace windows with it, as I use my computer for so many other things (music creation, photo editing, occasional programming), but would consider a dual boot if it made performance sense, however, it would definitely help to have a cheaper/free option when trying to convert my console loving friends to PC
I would like to echo Macro's sentiment that Gabe IS out to make money, and not doing it solely for the love of us gamers!