The exp bonus you get scales based on the dungeon you got in the roulette. At level 79 getting sastasha can just as easily get you just as much exp as a Mt Gulg run. (Both hovering around 10-15mil)
But on topic, I've said it before in the past, but unlocking all skills at lower levels just unlocks a balancing nightmare for the developers.
'Potency scaling' would either take way too much effort, time and planning to get right, or it would create a situation where leveling up actually makes you weaker in lower level dungeons, or sheer ridiculously OP (even more so than now).
If you want to implement potency scaling, then what milestone would you balance it around to give parity to the newbies who only have 2-3 buttons at their actual level?
If you balance the scaled potency at around 90% of your classes's capability to be on parity with newbies, you'd literally make a massive chunk of the player population do even less than if they only had access to 2 buttons, since a good chunk of the population can't execute their rotations properly.
If you balance it to be very forgiving on the rotation to meet parity, then you'd have dps that have even the slightest shred of a clue of their optimal rotation doing 2-5x as much damage at lower levels and completely obliterate even the slightest sense of difficulty any low level content might possess. Not to mention, having newbies would be an active hindrance if people were wanting to get in & out as fast as possible.
Like, compare a level 80 WAR to a level 15 MRD. The WAR would be capable of not only using their entire mitigation kit where the MRD only has rampart. The WAR would be capable of using a full almighty IR Decimate chain paired with Nascent Flash for stupid OP levels of lifesteal where the MRD would have none. Heck, even compare a level 80 WHM to a lvl 15 CNJ. The 80 whm has access to all kinds of OGCD heals, regens, % mitigators and aoe damage spells the CNJ doesn't. Potency and skill scaling when handled improperly would create a system where having newer players would be an active hindrance, and history has shown time and again that such systems can easily be open for abuse.
While the level sync isn't the greatest system, it achieves its job on creating parity for the most part, the only outlier being stat syncing. A better solution would be to maintain parity by having jobs learn a lot more of their skills at lower levels; I've always vouched for the idea that a job's core fundamentals should be finished by level 50, with 51+ stuff only being additions/changes/etc to the core rotation. Also every job should have aoe by level 15 tbh.