Not that I don't agree with you in theory though, since clearly you can expect at some point that there could be a problem with hardware driver support. Good news though, is that as Android matures, the hardware driver requirements should/will_hopefully stabilize to the point where the older drivers will be reusable on newer versions. What always has been open source, and will therefore continue to be useful for future updates, are the kernel/driver glue. We can thank GPLv2 for that. This open source driver glue was very useful in the initial attempts to get 2.2 running on older hardware -- by using the 1.6 driver blobs.
Now Android is a very different case to general desktop linux. Google has arranged it as a commercial linux desktop and has managed to be extremely successful with the platform. One of THEIR goals is to make hardware support EASY, so having a stable API/ABI is much higher on the list of priorities for this distro than for other distros.