This has been discussed to death, and it boils down to how would you handle it, so by parts:

-If you just enable old skills in old content, then the content becomes even more pointless than it already is, nothing in Copperbell is built to withstand a buffed double Enshroud, even with level 17 stats, much less the absurdity of PCT's burst, you wouldn't even be able to finish your burst in those cases.

-If you rebalance things to account for the high potency skills, then your Sprouts will feel completely pointless in the dungeon, as their barely-existent 1-2 combo can barely scratch the enemies, this includes if we assume the system is scaling, as you could still get 3 level 100s with 1 sprout, with the high levels slaughtering everything while the Sprout can hardly dent it, your best hope is for the sprout to not notice something feels off.

-If you add a scaling Echo buff so Sprouts can deal comparable damage, and let's give benefit of doubt and say that it's calculated appropriately at every step of the way from minimum dungeon level to 100, then you put level 100 players in an awkward spot where they need to put in significantly more effort to keep up or being useless, you could say it wouldn't matter since they'd be doing their normal rotations anyway, but it would make minor rotation mistakes much more apparent.

All options cause issues and it becomes a "pick your poison" situation, but if you would allow me to give an anecdote from another MMO...

So The Elder Scrolls Online, I played it for a while, it has a system called One Tamriel, it is supposed to make the entire game operate at max level, so a level 1 character and a level 50 character can do the same content, with a scaling buff that enables the low level character to keep up, there is some limitations to it, but that's the gyst of it, I started playing ESO after the One Tamriel update... and I find it utterly miserable, it's sincerely one of the worst experiences I ever had leveling in a MMO, it creates a bizarre situation where you can feel yourself getting weaker as you level up, as the buff starts to wane, and a level 1 character will have notably more HP than a level 50 non-Tank character(roughly 22k vs roughly 16k), since a non-Tank won't be built for durability while the level 1 is buffed all-around. Mind, I am a weirdo in that regard, and I actively enjoy the leveling process in MMOs, I always take the scenic route and see it through, someone else might find it appealing, but it's just something I would never want for XIV, even in a limited capacity.