The counterargument is that RISC architectures end up needing to use compressed instruction formats (e.g. ARM Thumb) because their clean orthogonal instruction encoding is so inefficient (e.g. it wastes a lot of instruction cache). If you already need the compiler to output some compressed encoding that your processor is going to uncompress at runtime, well, why not keep that uncompressed format as an implementation detail that you can change in future models of your processor?