You're forgetting Vista. If they hadn't of dropped the ball on Vista, which was a combination of small issues and not a bad OS overall, XP would have gone quietly into the night just like 2000 did.
Besides, sticking with XP for years was not a great idea. Same deal with old versions of Internet Explorer. You don't hear people making the same excuses that we should all be using a version of IE circa 2002 and that if they come out with a new version this year we wont need a new release until 2018. I think the only reason people complain about getting a new OS every 3 years and ALSO complain when they don't get a new web browser every three years is cost. If they have to buy something they will cry like 3 year olds now matter how awesome it is but if its free they will eat it up even if its just a bugfix with a new version number.
Just remember; Windows 7 is just Windows Vista after paying attention to some angry costumer feedback, and most people love Windows 7.