It is not that difficult to re-target a micro-architecture (what you call implementation) to a new ISA, especially if the ISAs relatively close, as most modern ISAs are. RISC-V and Arm ISAs are an example where that should be especially easy.
Why pay for ARM if you can retarget to RISC-V and still retain most of the benefits of your designs.
Right now, the Arm ISA has much better micro-architectures than RISC-V, why bother with RISC-V? I think this will change over the next 5 years. One of the ways to change this could be Arm selling RISC-V implementations.
Of course there are also dangers in RISC-V: one is that we will get too many RISC-V versions, so no stable software ecosystem emerges for any single one of them. Adapting Linux, GCC and so on to your specific version of RISC-V (or any processor really) is rather expensive.
It is the fundamental basic business ideas that they have it all wrong. Which is why every time these discussions came up they are non-productive. It is no different to saying Linux would take over desktop in the 90s because it is free. We are in 20s and coming up 30s. It is still not happening.
But at least ( or hopefully ) we have passed stage where RISC-V is fundamentally better ISA than ARM. And talk about the business aspect of it.
Edit: Ok I was wrong, the ISA debate is near the bottom of the page. I didn't invent the term "Riscy Silver Bullet" for no reason :).
The desktop? Maybe not. But Linux has done well with laptops in recent years (Chromebooks). Definitely not "taken over", but 30 million units a year is nothing to sneeze at either.
Completely agree.
The trend with RISC-V suggests that ISAs are going to be commoditized and the real value will be in the implementations themselves.