Keep skills when level sync’d

[Seraphor]

Your level 80 kit is significantly more powerful than your level 18 kit in Sastasha. It's three times as potent, before accounting for stats, just raw potencies.
So yes, you would wipe the floor with a party of level 80s while newbies would definitely increase the duration of the instance by a significant amount. Then we'll get to level 90, and 100 and it'll be insane.
Balancing this would be a nightmare, and entirely unnecessary, because they already have a perfectly fine balancing system, it's called level sync.

The solution isn't to keep your level 80 skills when synced down. It's to better distribute skills throughout the levelling process so that you're not stuck with single button spam for the first few dungeons.
By retaining your skills when synced, you will just allow them to ignore this aspect entirely, and early game levelling will just get worse and worse.

The level sync system is one of the crown jewels of FFXIV, that allows them to populate low level content, at a level playing field, despite constant progression and level increases. Messing this up would be the worst mistake this game could make.
You might think it's boring going from level 80 content to level 20 content, and you're not wrong, but just wait until sub-level-50 content becomes irrelevant or prohibitive for newbies and the game flat out dies because of it.