The problem is not the idea itself, but rather its actual implementation.
Suppose you get a DRG in Satasha, a level 15 dungeon.
At level 15, a DRG(or rather, a Lancer) does not have any AoE skill.
Now if the DRG is level 50 he will have more tools to increase his dps, such as Blood for Blood, Doom Spike, and Dragonfire Dive.
He will have even more at level 60 and 70 (Geirskogul, Sonic Thrust, Nastrond etc.)
How do you propose the devs balance these skill? Do you simply just reduce their potency so that the total output remains the same as that of level 15 player (which is simply impossible by the way as they don't even have AoE skill) making higher leveled player perform more for less?
Do you leave thing as is? Giving the actual level 15 tank much harder time to deal with aggro, possibly turning more people away from tanking? Hell it might even cause higher leveled players to despise lower leveled players even more for not having all these sick skills, making the run longer, on the other hand.
You said the current system is unbalanced, but what you suggested takes it to the other level.
I'm saying as someone who doesn't really enjoy level syncing much (mostly for just sub level 50 contents), but it is what it is.
So you either deal with it, or suggest an actual realistic solution.

Originally Posted by
Jojoya
The only MMO I've played that does it is WoW, and it works badly there. They mess with the content tuning so it either remains trivial for max level players being scaled down with full toolkits or ends up too difficult for players who are at the intended level but only have partial toolkits. I'd not enjoy seeing it repeated here. If the content is designed around players having access to only certain abilities then it should be experienced that way when synced.
I'll have to second this statement. The dungeons in WoW, while fantastic and completely stomped on XIV's one, have problem due to its level sync system. New players might as well not bother and just watch the veterans mow down everything in their path. It's part of the reason I did not bother continue playing.