A nit:
> For example, I have not encountered hardware for reading vertex attributes or uniform buffer objects. The OpenGL and Vulkan specifications assume dedicated hardware for each, so what’s the catch?
That is not my understanding of those specs (as someone that's written graphics drivers). Uniform Buffer Objects are not a "hardware" thing. They're just a way to communicate uniforms faster than one uniform per API call. What happens on the backend is undefined by those specs and is not remotely tied to some hardware implementation. Vertex Attributes might have been a hardware thing long ago but. I'm pretty sure there are older references but this 9yr old 2012 book already talks about GPUs that don't have hardware based vertex attributes.
https://xeolabs.com/pdfs/OpenGLInsights.pdf chapter 21