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.

Kind of, that isn't what many FOSS advocates of RISC-V are expecting, yet that is how many things will go, if it ever reaches ARM adoption levels.

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.