I've made yet another masterpiece today, which visually showcases the problem:
Chart only has damage as Y axis, skill expression is distance between skill ceiling and skill floor
Once again, we have Bob and Greg. Bob isn't very good and Greg is kinda decent. When Bob plays RDM, he does significantly less damage than Greg. He puts in less effort, so that's fine, right? But what if both Bob and Greg switch to SMN? Greg still deals very similar damage, while he does not need to put in as much effort. What about Bob? He deals significantly higher damage, even though he puts in same effort as he did when he played RDM. It's so significant, that the difference between him and Greg is very tiny.
So why would Bob go back to RDM, when he can just relax and play SMN and deal more damage? Why should Greg keep trying his best while playing RDM, when he could just play SMN and deal same damage for less effort?
Both jobs must deal similar potential max damage, otherwise they will be getting kicked out of parties. So there are only 2 ways how to make both RDM and SMN good picks. You either make RDM as brain-dead as SMN (I'm sure RDM mains will be happy about that) or just simply allow more skill expression for SMN. This will make Bob deal less damage, however that is simply fair.
I'm not putting in my personal suggestions how to increase the skill expression for SMN, but it does not necessarily mean drastic changes or adding button bloat just for the sake of adding bloat. There were some good suggestions from other people however.