Why would you assume that? The set of required messages for drivers will increase over time, adding power saving commands, new hooks for higher level Android bi

Why would you assume that? The set of required messages for drivers will increase over time, adding power saving commands, new hooks for higher level Android bits, and so on. "Your WiFi only supports Android Kernel API 3.9. You need 7.0 to run this version of Google play". The Linux kernel already has a stable user space ABI. If your hypothesis was valid, you would be able to upgrade the Android userspace on an old kernel with no problem. But you can't -- and that's with just one well specified ABI with a standardized, portable subset to worry about.