Just to check: Can Google unilaterally do this? Obviously they could keep the actual browser updating, and they could keep much of the OS updated, but do they own the whole stack, including firmware, enough that they can maintain the whole thing? (I think the situation is actually pretty good on x86 at least, but worth checking.)
I don't know about arm machines, but Google has control of pretty much the whole stack and end of support isn't based on technical details: models are dropped from support based on time from initial release, not based on the cpu that's in them, and there's not much hardware outside the cpu to have become unsupportable.
And cpu-wise, the only important thing that some cpus might not support is virtualization extensions to run Android apps; but I think that already has the right hooks to hide itself if the processor doesn't support it (or maybe it's just a list of models that are allowed to use it).
