The way they go about it is that it's not that this specific change would be OP by itself, but that since the job as a whole is considered OP to begin with (mainly because people complain about it) any addition to it would make it even more OP than before.

RDM has suffered from this for years upon years, where this type of things are given to other jobs instead because we were considered OP at the time. Even if the skill or spell in question was anything but that.