Using his MNK example, what if the MNK in question is only 40? Applying a universal multiplier to bring down a level 80 MNK would make a level 40 MNK deal close to half the damage of a level 15 MNK, even playing absolutely flawlessly. "nerfing damage" is far from a simple concept, as it requires a huge amount of balancing, analysis of toolkits at literally every level that a job gains a new part of its toolkit, and would be a logistical balancing nightmare; you'd literally be getting weaker in older dungeons as you leveled up then when you were at that level, quite the oxymoron.
Unless they go dungeon by dungeon, job by job, meticulously nerfing damage by EXACT amounts for EACH instance in the game, damage nerfs will always either:
1) cause the 80's to still be overpowered if they balance around the 80 not doing an optimal rotation, meaning any 80 doing an optimal rotation will still utterly shatter any level 15 out of the park.
2) cause the 80's to have to perform flawlessly to match the limited toolkit of a level 15 if they balance around performing a near optimal rotation to keep up, meaning by far and large with the skillbase of FF14's players, instances will become longer/harder for basically no reason.
And how do you balance tanks and healers? Think About it. a lvl 29 GLD will never have Hallowed Ground from a level 80 synced PLD. a 22 MRD won't have the lifesteal of a level 80 WAR with Nascent Flash. A level 80 synced WHM would have benediction; where the 43 WHM wouldn't. You think they're going to spend and take the time editing & balancing ALL those skills at EVERY level too?
And if you'd have to do ridiculous changes to these abilities to the point they're no longer the same ability (Like doing something absurd like turning Hallowed Ground into 1% mitigation, or Benediction into 10% heal), then...what was the point?. If you have to utterly gut these abilities to make sure balance is upheld to the point they may as well not exist or no longer resemble the real ability, then it's not the same toolkit anymore, and completely invalidates the point of trying to create a system that lets you use all your fancy new abilities at lower levels.
There are far too many variables, far too many things to keep track of for any 'simple' nerfing solution to work if they want to maintain balance. Heck, some jobs simply won't even have AOE at level 20 whereas the same jobs synced down from 80 would, meaning your dungeon literally takes longer because you had a low level character in your group.
Oh, and the fact that each expansion basically invalidates all this balancing meaning they'd have to do it AGAIN for every expansion. Yeah, I can only imagine how wonderful that sounds to the devs.
There's simply no chance they're going to allow skills while synced, since they've made it clear they want every 22 GLD to be on the same footing as every other 22 GLD, skill wise. An actual realistic scenario is an ability level squish, pushing more of the later abilities into lower level ranges. That way every player of the same job has equal access to the same skills, the amount of skills at lower level and thus the speed of combat is increased, only without an absurd logistical nightmare of balancing issues plaguing it.