What you aren’t comprehending is that American *companies* (not software firms, but brick and mortar sell-you-a-car, make-your-pipe, mill-your-paper companies) have all written in-house software to run their businesses.
Vista can’t run most of that software, even though XP can.
These companies don’t go rewriting their in-house software every time a new OS comes out. That would cost *money* to deliver the same functionality, a very bad use of corporate resources.
Likewise, they aren’t going to replace all of their PC’s every 4 years. Heck, they’ve got to keep the old PC’s around just to run XP and their in-house software, now.
You are living in a different world. For you, it might just be one PC to swap out every 4 years...but to a company, it’s thousands of PC’s and thousands of employees who have to be trained and scores of system administrators who have to deal with the old, and new, and the networking interfacing of old and new PC’s and OS’s...which is to say, the less changed, the better the company runs (for less money and less downtime in swapping out machines, too).
Where we worked they changed out the computers every two or three years. If the software makers don’t want to update their software they deserve to go out of business anyway, just like the buggywhip manufacturers who didn’t move into the modern world. Four years for a PC is plenty of usage anyway. It is functionally obsolete.