Well speaking as a player who has played in Sastasha with max lvl classes it doesn't annoy me much that I can't use most of my skills. Do I enjoy it? Not really but it doesn't bother me enough to annoy me. I understand why the lvl sync system works the way it does, and I like that it puts players on similar footing in a dungeon instead of having sprouts watching all the content being blasted for them if they happen to be grouped up with players who are far higher lvl.
Most players want to be able to participate in the content they're doing, and allowing players to have their full arsenal of skills in even the lowest lvl content would achieve the opposite of this because things would die so quickly. Even with the damage output adjusted for the content having major cds, ogcds and aoe would be enough to trivialise so much. Sastasha doesn't have mobs designed to endure lots of aoe because at that lvl players have little to none. Putting in players with strong aoe in there would make the content an absolute snoozefest. You think it's boring now? Well it would be even more boring if the mobs fell over in just a few aoe hits.
I can tell you that if FFXIV allowed players with all their skills blaze their way through low lvl content in DF then I would have unsubbed before I got to lvl 50. I want my participation to matter, not have it snatched away from me by a far higher lvl player with an enormous set of skills compared to me. I don't want someone else to kill everything before I even get the chance to feel how my class works in group content. I don't want to be boosted.
One of the main reasons why I quit WoW was because the lower lvl instanced content at the time was incredibly dull. Things died with little effort. I barely had the chance to actually use my class. It made lvling up excruciatingly tedious. Had I encountered the same thing in FFXIV I would have moved on to a different game very quickly.
I am not saying I found Sastasha challenging even when I first did it, but the mobs didn't fall over so fast that I felt like I could go afk and little would change. They die faster now but still a lot slower than the low lvl dungeon mobs in the state WoW was in before I quit. The content still requires some amount of attention from everyone to be done in a timely manner.
Also the topic of players not knowing how to play well in high lvl content is very frequently discussed here. Well if lvl sync did not remove abilities from higher lvl players, then sprouts would have even less opportunity to learn how their classes work due to higher lvl players trivialising content for them. It would result in even more players not playing at an acceptable standard at higher lvl. Some people here really need to look at the bigger picture.