That was my idea for Win7
I beta tested Win7 and worked primarily on the backwards compatibility issues. The test bench I ran Win7 Ultimate on would not even run Vista Home Basic. Before the RC came out I was running hardware, and some software (games mostly) that worked in Win 98.
The big problem with Vista was the lack of computability with hardware and software that worked on the XP machine just before they "upgraded" to Vista.
So yeah, backwards compatibility is a big issue with Windows users, both consumer and businesses. Would you want to have to purchase new printers, monitors, routers, switches, and production machines every time a new OS came out?