The problem with balancing around average DPS is you have higher highs and lower lows. The higher you level, the more access you have to higher potency actions, which just pushes damage higher and higher.

To illustrate this, PLD has access to 1 AoE at level 15, Total Eclipse, which does 100 potency on every enemy hit. By level 80, we have access to Confiteor, which is 1000 potency, it does 10 times the damage of Total Eclipse, 1 GCD of Confiteor is equivalent to 10 GCDs of Total Eclipse. You cannot really scale this effectively without it either being either insanely strong or having Total Eclipse being really weak, this is before we even look at the rest of the blade combo, which includes another 1000 potency is Blade of Valor and Blade of Honor, which also happens to be an oGCD.

Which then brings up oGCDs. How do you balance them? It is free extra damage that is another layer you have to work around.

How about those actions that get upgraded to AoE? PLD has 2 in Expiacion and Imperator, again, free extra damage.

How about healing kits? Low level healers have to rely on the GCD to heal, whilst high level healers rely on oGCDs, leaving more GCDs for more damage and you cannot really 'estimate' the amount of healing you are going to do on the GCD to balance DPS, because that is also reliant on the tanks defensives, which brings us nicely onto;

Tank defensives. Holy Sheltron has a heal, PLD's magic, has heals, this just forces healers in lower level content to ignore healing even more, not to mention just the upgraded defensives in general. Sentinel to Guardian? That is a spicy barrier which no low level mob is going to be able to break through.

There are way too many hurdles to get through to make it worth investing in. They would be better off making the low level kits more engaging and making more traits like 'melee mastery' to just increase the potencies of a large number of actions at the same time. The new player experience matters and this is a better way to do it than allowing high level players to just curb stomp everything in one hit.