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 will no longer bank AP in order to boost your Melee, but building into Armor and the improved Mighty Tank will fill that AP void to an extent. You will no longer be contradicting your build as you try to make and play it.
While this may be an imperfect solution, it was the best we could do. In an ideal world the Concussioner set bonus would likely be overhauled, perhaps in the direction of the Melee scales with Firepower.