You are correct that melee scales with AP.
However, the set scales with Armor and damage taken.
The reason Concussioner didn't work as an AP set was because Earthen Shell denied the set bonus from storing damage, so it didn't really work at all given Earthen Shell's constant uptime. On top of this, non of the apex Pax nodes worked well with Concussioner, which was a design oversight.
We tried to make the set work with AP but there was always a problem where players had to choose between committing to AP and Melee OR Armor Nodes, so you were always forced to created an unoptimal build.
We made the decision to make it FP because it was a low cost solution that allowed us to resolve the Earthen Shell + set bonus problem and finally make an Apex Pax node work well with the set (OverForce). This change will players focus more on Armor than trying to invest into double Melee damage nodes with some AP.
You're correct that you w...