Earlier quoted context omitted.
> It doesn't have to be a mainstream kernel. The only important part is that it is a fixed kernel version This is how it is done already. Google picks a kernel release (historically 3.0, 3.4, 3.10, 3.18, 4.4 etc.) and ports Android patches to that kernel while developing new features as well. Then SoC vendors do their bringups using that kernel tree.
My point was, kernel version should not change with every Android release...
A device is released with a specific kernel version and it doesn't get upgraded in devices lifetime even if the same device gets several Android upgrades. Native Nougat devices released in 2017 have kernel 4.4 which was released in early 2016, which is not even considered a new kernel by the upstream standards.
What do you expect the benefit everyone gains from sticking to the same kernel for 5 years? What is the point of developing a SoC in 2017 and bringing it up using Linux 2.6.32 instead of Linux 4.4? What is the benefit for Quallcomm, for Google or for users?