Remember when I said 'developer balancing nightmare'? What you said is literally the reason why it'd be a nightmare.
How would you change the high level abilities so that they'd still be useful with their new numbers, but not completely useless due to synced potencies/numbers? Think carefully on it for a minute.
Lets go with WHM quickly. a level 15 AST has zero regens. For regen to not be OP and completely outright crush everything that level 15 AST could ever possibly do, you'd have to nerf its potency so much that it'd be literally worthless to use,
completely defeating the point of implementing potency scaling. Lets go with another. WHM's wings ability, which currently grants 10% damage reduction. If you nerfed it to say 1%, then what's the point of it even existing anymore at that level? 1% is going to do basically nothing, so what was the point of implementing potency sync if it may as well not exist? How about benediction? How would you modify a 100% full heal to meet parity with a level 15 AST without making it useless?
Also how do you balance jobs with AoE at 80 vs jobs with no AoE at level 15? like DRG. Having a LNC would always be statistically 100% worse all the time, since having AoE > no aoe. Unless you want to also nerf the DRG's aoe potency to the point it'd be useless to use which again calls into question the point of even implementing potency sync when half of your skills would be useless/nerfed to the point of non-use.