Unfortunately, it's not as simple as that.
If you look at the stats, the XP base has been falling at a steady rate of about 2% per month, so your XP base will continue to decline quickly if it's in line with the general demographics.
Furthermore, o/s version and the age of the hardware it's running on, tend to be closely linked, as most people (and businesses) don't perform major version upgrades on hardware, they replace hardware with new versions loaded. XP's age tends to suggest that the hardware is well due for replacement in a large proportion of cases (even if they were downgraded from Vista) and MS are about to release a shiny new version with interesting new hardware options supporting it... expect the XP base to take a further big hit as a result.
Don't underestimate IT departments. We buy new Windows 7 machines, wipe them, and put XP on with our volume license. I really hope that the pending end of security updates forces us on to Win7, but I think we'll keep using XP until hardware vendors stop making drivers for it.