Do you really not see how ridiculously 'tinfoil hat' a fear such as that is? Such a scenario would not ever eventuate because, straight and simple, it's about the only thing which could happen which would end up with an outcome in which the 'PC world' isn't a 'Windows world'. Should MS even contemplate trying such a move, to ensure its acceptance amongst their customer base they'd need to reduce the asking price for the new version to a pittance. They'd never be able to even recoup their development costs. It's a silly suggestion which isn't even a remote possibility, really!
I'm certainly not a tin foil hatter. But, it is actually more plausible than you seem to think. If you follow the legal rumblings, the various departments of justice (not just in the US, but worldwide) seem to be gearing up for another round of anti-trust suits specifically over Vista. If you recall from last time, the anti-trust solution that was most popular was splitting Microsoft into two companies: an OS company and a services/products company. Such a move could severely and directly impact how these servers operate. In fact, it could necessitate shutting them down. Again, like you said, unlikely but still _possible_. It's the possibility in and of itself that makes me uneasy about this.
It's pretty much the same principle that I find fault in Steam from Valve. Right now, I've got ~$250 in software that I've only been able to buy using Steam (granted, I bought them in the store but they required authentication via Steam). Should Valve go belly-up, I'll have no way to authenticate my games and I lose that investment. That makes me more than a little uneasy.
I understand what you're saying - "the likelihood of Microsoft making such a move is so unlikely as to be not plausible." However, that's putting an _awful lot_ of blind trust and faith in a company that seems to do whatever it likes and then hides behind the guise of "piracy prevention." When it comes to my OS, I'd rather not have to put blind faith in whether or not it'll function. Hell, I'd be OK with the Vista style of phone-home activation if it was just in Microsoft Office or other Microsoft user apps. But having it in the OS is what makes me uncomfortable.