>I don't really see how the ISA is relevant

There's definitely better and worse ISAs e.g. some are unjustifiably complex.

Yet the key advantage of RISC-V is not at all technical, and lies in its licensing scheme.

There is no need to ask for permission to implement RISC-V and thus be able to leverage the best ecosystem out there, which RISC-V is rapidly building.

There is also no need to ask for permission to extend it with custom extensions, to suit your specialized needs.

Using the RISC-V trademarks is only a little more complicated, as it requires proving standard compliance.

Yet this aligns well with most implementations; Compliance is desirable as it enables tapping into existing software and tooling, which is much cheaper than building and maintaining bespoke solutions.