Most users will give Vista the finger & go straight to W7, despite the fixes in SP1.
At any rate, 3-5yrs is not a particularly long cycle for any mass adoption, because IT has to run the OS through vigorous testing before they adopt .. and quite frankly, a Dual Core PC, with a 32b OS and 3GB Ram is more than sufficient to check email, browse the internet & run Word & Excel.
Why the expense to upgrade to a new PC & OS, when the existing is fine?
Games are driving the PC upgrades (and hence OS upgrades). Business would prefer to just stay exactly the same, and fix bugs