Yeah, the extensibility of RISC-V that allows implementations to scale up and down on features, including non-standard ones is really cool, but compared to other ISA I think the biggest improvement is it being Open and royalty free (except for trademark+compliance certifications)
I think you want that, but in a reserved instruction space with mandatory fallbacks.
It's how things work anyway, things that are on the edge of what can be done are good for running business and maybe not finalized enough to become standards.
It'd be cool for it to be copy-left, but at the same time I guess it could end up being too hard to get business running as the apparent risk of a competitor just copying your product would scare investors and that would end up in lower availability of actual hardware you can buy.
- https://riscv.org/about/faq/ j - https://riscv.org/membership/ (I thought this was going to be expensive, but 2k USD/yr -> 5kUSD/yr after 2yr can keep a small company going)