That doesn't make any sense. Each level's DPS value is a number. Compare those numbers and you get a ratio. It's pretty easy to make them equivalent. If you multiply the level 80 rotation by 0, the level 15 rotation is a lot stronger. No matter how big the high level DPS value is, it can always be made equal to or less than the low level one.
Will all jobs have the exact same multiplier? No, but using the same one for all jobs in a role is pretty reasonable. If that didn't work it would mean that classes were very imbalanced. Also keep in mind that in leveling content perfect equality isn't a requirement. As things are now, in a given dungeon one class might have AoE while another doesn't. One player might have role skills while another doesn't as these don't get synced. A common multiplier for all DPS, all healer, and all tanks is perfectly reasonable.You can't just add another x0.33 multiplier to make up for this as:
A. Not all jobs rotations will have the same disparity, so that extra multiplier will not fit all.
Level 15 rotations aren't foolproof. An ice mage does about 50% of the damage a proper BLM would do. If someone at level 15 just mindlessly mashed their ranged attack as a melee (which has happened) they are also going to do about half of their damage. You see something similar when a class is low enough to lose their AoE. And of course whether you level sync or keep skills could be a toggle. Lazy players, which are rare enough, won't be a bigger problem in a dungeon with stat sync vs level sync. Poor or new players might actually perform better since they won't have to deal with constantly losing out on new skills and be able to practice with their full ability set every time they enter a dungeon.B. We then get the problem of a level 80 character needing to pull off a level 80 rotation flawlessly in order to keep up with the single button spam of a level 15 character.