Yes, you can create your own ISA. But to run what software?
If I create my own RISC-V implementation, I can install Ubuntu on it. Maybe even Steam.
See the difference?
And, the market has responded with a tidal wave of CPU contenders. Like in the rest of the world, not all of them target the highest end portion of the market. But some are choosing to play there. Have you checked-out Ascalon?
And why did Qualcomm pay all that money for Ventana recently? You do not expect them to release high-end RISC-V chips? I mean, they already ship many low-end ones.