"Just solve it in decoder" is a perfectly valid approach for now, and if this turns out to be unrealistic then they can go ahead and add a conditional move extension or whatever it is.

It’s far easier to add things to interfaces than remove them.

One mistake they have made that may bite them later is their compressed instructions extension. It takes up far too much of the instruction set space for the amount of utility it provides, even on microcontrollers in my opinion. It also introduces lots of edge cases around word alignment across cache and memory protection boundaries.