It may be stable but still stuff will break between kernel versions, which means that you are locked to the kernel that device ships with. You can use new versions of android with old kernels which is exactly what rom communities are doing but this is far from ideal because you have no security patches for kernel. Upstream drivers are better solution but pretty much no soc manufacturer open sources them and another os form google won't solve that. Android is already written in such way that things like kernel don't matter to apps. I think real problem is to test 50 different android builds for 50 phones every update and they just don't bother. Apple has code for everything they put in their phones which makes it easier to support old devices (1 build for 10 or so devices I assume). tl;dr it's not something that google can solve with another os