Pointing to Halo just seems stupid, if that game had ended up coming to the PC and Mac like it was originally going to people would be jerking off over Bungie's little forgotten cult game with those big open areas you could drive a number of vehicles around in. Halo is kind of funny, because despite it being this huge hit, and other developers trying to make their "Halo Killer," off the top of my head I can't really think of that mean first person shooters that actually had stages and weapons like Halo. Most people seemed to take from Halo is the rechargeable shield as rechargeable health, which was talked about when it came out, but the other stuff, (and LAN parties) was what made that game. The same type of thing oddly enough happened with GoldenEye too.
The shift in the demo didn't happen because of Halo. It didn't even happen because of the Xbox. It happened because the PC boom of the '90s ended. By the mid '90 like every family (at least in America) had at least a family computer, and these were nice ones that could play most anything that came out. But by the 2000s, if they were even getting a new computer it'd probably be a laptop which wasn't built for gaming, (or at least anything that needed some power) and later still it shifted to phones.
The demo shift happened because there wasn't the audience on PC anymore like their had been just a few years earlier. Now maybe the XBOX quickened this more than if the console market was just the Japanese PS2 and GameCube, but it happens regardless because the PC just didn't have the audience it once did at that time. Only thing that changes is maybe more western developers go under, and the target console is the much weaker PS2.