I can hear the screams of anger now from millions of users who don't want to switch. On the other hand, few or none of them would stop using the net or even switch ISPs. Most would probably grumble, then switch to Vista. Hardware and third party software vendors and congressmen would back the ISPs because it would trigger the biggest mass upgrade since Y2K and create a surge of thousands of jobs.
The security debate to be acted out before congressional committees would be entertaining. We would pit the antimonoculturalists on one side versus the ban-those-Win95-skeletons proponents on the other side. It would also make the perfect opportunity to advocate the mobile browser plus net apps as the non-PC alternative architecture.
Could a major ISP successfully refuse Mac and Linux customers? I see no legal impediment. They can argue security and simplfied support as their motives. Once again, most aggrieved Mac and Linux customers would scream, but they would rather switch than go back to dial-up. Therefore, relatively few customers would actually defect.
I hate bringing up such an ugly speculation. I can see the flames coming my way now. But, the simplicity and rationality of a Vista-only future from the point of view of the ISPs and others seems too powerful to ignore. Perhaps the question should be, what would stop it from happening?"