RV64 has a maximum 32-bit instruction encoding

I kinda expected that to happen, since there's already enough to fit all required functions. So yeah, even this is not a good enough criteria for bit rating.

those original 8-bit intructions still exist, and take up a huge part of the encoding space, cutting the number of n-bit instructions to more like 2^(n-7)

err... they are still instructions, right? And they are implemented. I don't see why you would negate that from the number of instructions.