Remember when everyone asked for a nerf of SMN during ShB.
They took months if not a year before nerfing it by a mere 50 potency, which made it remain among the best DPS, just below SAM and MNK.

The "mobility argument" was always flawed from the start. E8S was the most mobile tier and physical ranged did not shined, it was the casters who shined.

If the chart is like this, it's because SQEX decided so. I made my peace with SQEX balancing and SMN will probably get a DPS buff that will once again throw the logic of balancing to the trash can.
And of course be ready for the SMN main whining that they deserve that buff because they have 4 casts per minutes.