I have no doubt that give 6-12 months, Vista will also be a solid useable OS that will be preferred over XP by all but a handful of people.
First of all, "the new one" should be preferred since the day it comes out. If the company does a good job, that should always be the case. The fact that it isn't proves that Microsoft has failed with Vista and is now only trying to fix its faults.
Second, I believe there comes a time very soon when XP becomes hard to get and its support for new hardware might also become worse as time goes by. You are giving Vista "up to a year" to be usable, but after that, is XP really even an option? People learn to deal with Vista because many of them have to.
To summarise; Microsoft keeps IT staff at work