Yeah, this is just simply a terrible idea all the way around...
Nope. Great idea actually.

bad for the poor Lv70s that have to do 10x as much work as the newbies,
Just nerf the abilities above level. Kind of how they nerf the stats of gear above the level of the dungeon.

and bad for the newbies who would get bored watching the experienced players doing their stuff.
Wrong, people would be further incentivized to get to the level they get to unlock the flashy stuff. It's literally the entire incentivization structure built into the leveling system in all RPG's in the first place. This just increases the potential of discovering the animations organically.

if there's a, say, MCH that just started, and I end up in their group as a MCH myself, they might be stumbling over their rotation trying to find the optimum way to use their reloads... well, the way it is now, they can simply copy what I do and learn a thing or two by paying attention to what I do.
Finally, a decent hypothetical. Sure, this could be an issue. Again, if abilities above the dungeon cap were nerfed to the point that using the at level rotation was more efficient, you'd be incentivized to use the at level rotation, but not forced. Even in this supposed hypothetical (which is decent, don't get me wrong) input and help is still up to the player. Only if the player cares enough in the first place will they either learn or teach. From the vast majority of Mentor threads on here and reddit and in game, this doesn't seem to be the case. Still, at least this was a decent hypothetical worthy of mention, despite a lot of the strawman and naysaying that's instead being done in this thread.


Nah the OP is saying to lower the damage output of all abilities.
Better to just nerf above sync abilities rather than worry about nerfing all abilities. This solves the issue of overpowering tanks and other players with a more dynamic rotation (which I'm not completely against, but a sync'd ability list that is level dependent solves this issue).

That Lv18 BLM beside you would do 20% of a mob's HP in damage with an Astral 1 Fire. You would likely do 20% of a mob's HP in damage with an Astral III Fire IV. Cast an Astral 1 Fire yourself and you'd barely tickle it with the OP's suggestion.
Not necessarily if above sync abilities were adjusted accordingly and at level and lower abilities maintained. Issue solved.