A potential rework to the level-sync system.

[Vyrerus]

That'd be true if Lv. 80 players are thrust into low-level content. That is not what I'm proposing though. Even if skill sync is removed per my suggestion, veterans will still be level synced and ilvl synced, which would still prevent one-shots and insta-kills.
And as I've mentioned earlier, if old-content is still a mindless stomp with or without skill sync, I'm pretty sure 99% of people would agree that being able to use all 24 skills would still be more satisfying and fun that being forced to spam only two skills over and over ad nauseum. So why not make something mindless even just slightly more fun?

Because I don't know about you, but I'm pretty sure most people wouldn't define pressing one or two buttons over and over in mindlessly easy content as "playing a job in a satisfying way," especially when the typical FFXIV player disables level sync when trying to clear old content for the first time via Party Finder or when trying to farm said old content for glamour/items/etc.

I wasn't talking about one shotting things. I'm literally telling you that if you give a person their level 80 skillset, they won't be able to use it half of the time anyway in low end stuff. The monsters in synced content die that fast. It's far more annoying to try to use your whole kit when most of it goes to waste, anyway.

Not too mention, if they caved on this sort of thing, the next thread would crop up, "Why not just give us our full kit right away at level 1?"

Also there are synced solo duties related to the Main Story Quest and Job Quests. While they aren't exactly pillars of difficulty, they are expected to be done at certain levels and be mildly challenging. What you propose would allow people to level past these points, and then go back and destroy them with skills not locked behind quests. This may potentially break some quests, because certain skills would allow for more player power than you're supposed to have from a design standpoint. They then have to fix something that would have never cropped up otherwise.

They also desire for the Ultimate fights to stay challenging. The level 70 Ultimates would become even less of an achievement to beat.

Your idea is bad for everything.