You are correct that you don't have to go to Dell to upgrade your copy of Windows but you sure as hell might need Dell to provide specific drivers for your hardware. The issue is the phone vendors need to make sure the hardware drivers are compatible with your new version of android before you upgrade or your phone might be broken. This is the reason so many phone lose a bit of function when you install CM.
I am not defending the lack of updates just giving the reason that Andriod is updated through the manufacturer and not Google. Unfortunately too many of the phone manufacturers would prefer to sell you a new phone instead of investing the time to test the old hardware with a new O/S.