Closed source firmware is one thing that actually runs outside the Linux system... but there's also the user space libraries that are needed to interact with the drivers (eg libgl etc... or the vendor partition in most Android phones)

I don't think anyone expects non specialized os images to run on this hardware. That would require a standardized userspace abstraction layer like the one Android has been building out. The kernel is just a tiny piece of what's necessary because drivers have effectively moved into userspace. Graphics is the only area that has embraced this properly in "desktop Linux"