The typical routine for a new PC is to install the OS, and then find and install drivers for the hardware you have.
With a phone, the hardware is often proprietary and different from device to device. In the PC world, you get your video card driver from the guys who sold the video card, but with the phone, all the hardware comes from the device manufacturer, so they'd need to be the ones supplying updated drivers.
And the incentive just isn't there for Samsung or HTC to spend money working on software that enables an older device to run a newer OS. Samsung makes money when you buy the hardware, and that's it, unlike in Apple's world, where they also make money when you buy software, so it's in their interest to keep your phone current.
One solution would be for Google to declare a reference platform and if your phone is based on the reference platform, then you can get Google's OS and install it directly. Problem there is the carriers don't want that.
So if Google is going to succeed here, they're going to need to sell a cheap phone ($199 range - you've got to compete with carrier subsidized phones) that's good enough quality that people actually want it, sell it direct, and support it themselves. Problem is, I'm not sure there's any profit in doing that.