Compare: WHM Lv15 casting Stone for 140p vs WHM Lv54/synck15 casting Stone III for 220p vs WHM Lv90/synck15 casting Glare III for 310p. The difference between low level and max level in this case is more than double.
Also keep in mind that even if you syncked the higher level player's actions down to the potencies that they should have at lower levels, each job learns new oGCD Abilities that insert into the rotation.
A RDM Lv15 only has Jolt, Verslowspells, Scatter, Corps-a-corps, and Riposte. That means for trash they can only dualcast ST-AoE, they have no AoE Mana spender, and they have no AoE oGCDs. A RDM Lv90 can dualcast AoE-AoE, spend Mana on the AoE Moulinet attack, and use the AoE Contre Sixte Ability, and also have Fleche and Displacement for extra single target hits.
A RDM Lv15 on single target only has Jolt-Verslowspell, a single melee Weaponskill for spending Mana, and Corps-a-corps. A RDM Lv90 can replace Jolt with Verfastspells, has a six step combo that includes three high damage spells, four oGCD free damage Abilities, a party damage buff, and an instant Combo Ready button. In the time it takes the lower level RDM to build up enough Mana for a single Riposte the higher level RDM can do an entire melee combo all the way up to the flower sword laser
and in finishing that combo they'll already have built up half the Mana they need for the next combo.
(I've used 15-90 to highlight the most extreme case but 50-90 would not be far off, as many jobs still lack their core actions at 50.)
This means that keeping higher level actions in lower level content cannot happen without one of the following consequences:
- The higher level player's attacks would be very weak compared to the lower level player's, to account for the additional actions and trait potencies, which means the higher level player has to do far more work than the lower level player to put out the amount of damage expected at that level
or
- The higher level player's attacks would deal damage representative of the amount of extra work their rotation entails and their damage contribution would be obnoxiously high, severely eclipsing the lower level player's contributions
Further, the average player doesn't excel at their Lv90 job to begin with, so in case (1) many higher players would actually do less damage than expected and things would go slower on average. If the devs compensated for this, then (1) in general would
also simultaneously be issue (2) for those players with higher skill at their jobs.