Humor me. How do you think you can keep a lv15 CNJ's output who only has Cure I and Medica in their arsenal balanced to a lv90 WHM who has their full kits? Do you force all heals into 450p heals? Hate to break your bubble, it's not that simple.
We have healing abilities that are not bound to GCD. Suppose that everything are reduced to 450 cure potency which is the base Cure I's potency, a synced lv90 WHM will still massively heal way more a lv15 CNJ. Any GCD Heal + Tetra + Benison in a single GCD all equals to a whooping 1350p GCD, which is accounts more than a tank's full health at lv15. This is a feat that a lv15 CNJ cannot achieve. You know where is this going to lead next: sprout CNJs will get kicked in favor for synced healers in order to perform wall pulls reliably (it's not that wall pulls in those low dungeons are impossible anyway nowadays... but that's beside the point). So now what are we going to do? Nerf everything to certain percentage so that no matter what the synced healer do, they cannot pump out more than 450p per GCD? Not so fast, my friend. Mikey_R explained & hit the nail of the head for this predicament, including the tank's side of balancing difficulties.
What I think some of us do agree in this thread though, yes, lv sync does makes some of our jobs' gameplay felt incomplete and it can feel suck to play. The most glaring ones are the nonexistent of some jobs' AoE buttons until second half of ARR's level progression (looking at you DRG) and the lack of anything to do. One of the few ideas that other forum dweller had mentioned before could probably be implemented. Such as specifically giving WHM the low lv version of Assize, Temperance, Benison, and Aquaveil that could encourage learning healers to weave abilities while giving them something to do. Basic AoE rotation too for all jobs perhaps, like Deceptus mentioned.
Full lv90 toolkits though? Sorry, it just won't happen without breaking something and creating more problems.