Considering Potency is just a damage multiplier and this still doesn't account for different healing and mitigation kits or oGCDs as a whole, this isn't a solution to the problem.
They obviously take it into some consideration as jobs aren't that unbalanced compared to each other at lower levels, let's use your examples.
GNB avergae potency per GCD is 295 (123+ BS), add in Danger Zone for an extra effective 20.83 PPG for PLD Combo 270, for SW + CoS, 43.3 PPG, so, add them up, GNB has a potency per GCD of 315.83, PLD, 313.3. Very similar. GNB does outdo PLD in mitigation/healing however.
DNC Standard Step takes 4.5 seconds to cast, which, if you reduced to the equivalent of a 2.5 second cast, is equal to a 400 potency per GCD. If you take the average of the DNC GCDs, it does come to ~200 PPG, not counting Fan Dance. This might seem high, however, Ninja can use Raiton at 650 potency per GCD (2.5) and they use it twice at level 50, one of them under Kassatsu for an extra 30%. As a reference, Ninja's adjusted combo potency is 384.3 potency per GCD (2.5). Granted, DNC will have an extra 5%, so you can increase the potencies to 420 and 210 respectively. And do not get me started on SAM. All this to say, it isn't good enough to just go, look, high potency, there is alot more that goes into it than that.
As for AoE, The falloff for Standard Step is insane. The falloff damage is the same as Windmill, the first step in DNCs AoE combo, which means the AoE combo would do more damage than the falloff and the only reason it is so strong is that initial hit on the one enemy. Alot of jobs will be able to out do that.
It got an extra ~3% on Afflatus, yay. The bigger impact would have been reducing the lilly cooldown from 30 seconds to 20 seconds, so I don't know why you didn't reference that.
The point is to balance the jobs (roughly) around each other and not have one glaringly stand out from the rest, which I would say they have achieved. It might not be balanced perfectly around the content due to system changes, but the point is they are balanced around each other. Now, you cannot balance a high level job with a low level. You cannot just make the lower stats do all the work as the potency will just inflate the damage. However, there is also no easy way to adjust the potencies to allow balance between the jobs. This is the point. If they wanted to rebalance the older stuff, it would probably be a simple case of increasing stats by, say, 20%-50% for ARR. Wide range, I know, but that would be an easier thing to pin down than the insane prospect of potency balancing every job for every piece of content for every level range.
The question was never whether it could be done or not, the issue is would it be worth the dev time to do it and the answer is going to be a no. If you think it is an easy thing, balance one job, from level 90 to all levelling dungeons. I guarantee you will struggle to find any universal solution to the problem and now think, you now have to do it for the whole level range from 15-90 to go into any dungeon from level 15-90 and now you have to do that 18 more times, once for every job. Again, it is never a case of whether it could theoretically be done, it is a case of, would it be worth it to even do in the first place.