A potential rework to the level-sync system.

[Intellion]

And as far as the OP goes, I understand wanting to not be bored in low level dungeons, but what will wind up happening is people killing monsters so fast, that a lot of jobs can't be played in a satisfying way.

Think of deep dungeons. Up until the 101+ and 31+ floors, monsters die so fast in them, that you never get to use all of your abilities until the boss floors. This remains true for a long time in both, and it makes them really annoying to play in, at times. For instance, Wildfire on MCH can barely be used on trash, even into very deep floors, without actually detonating it early manually.

It's better if we just take it in stride, the old content dies so fast anyway.

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.