Perhaps I don't see how this all fits together. I am interested.
Each SoC differs in varying degrees. And is integrated into boards which differ in varying degrees. Which connects to a variety subcomponents.
Just about anybody who uses a board ends up being a kernel developer, writing SPI driver code if nothing else. There is a huge variation in skill. I don't see how a single subarchitecture can ever keep up with the growth in variations.
I would be happy if each variation was a branch, with branch maintainers.