> Why wouldn't it get away with doing just that?
Well, if a MS OS was based on a subscription model, presumably it would depend on people have broadband access to pull it off, which is far from universal yet, because the OS would need to periodically check to see whether you were paid up or not.
I doubt that people will accept this. You buy a PC from Wal-Mart or Dell for $600 and then have to pay MS ~$100 a year to use it???