Thank you very much for this. I share these exact sentiments, but you worded it much better than I could.
To be quite frank, people argue against this topic by bringing up balance, challenge, potential newbies getting discouraged, and so forth. However, what I actually observe in-game is the exact opposite of these claims.
1.) Balance? Even with levels, ilvls, AND skills currently getting synced down, older content is a mindless faceroll anyways. There is no more challenge whatsoever since even synced players are ridiculously strong, and whether they're veterans or sprouts, the common sentiment is that everyone just wants to get these old content over with in the first place; veterans just want to get out ASAP for the tomes, and sprouts just wanna get it over with ASAP to proceed with the story. Never once have I seen anyone, sprout or synced player, ever say things like, "Hey tank, can you please slow down? I wanna fully explore Copperbell Mines/proceed as originally intended" or something like that. And when you look at party finder and see sprouts or latecomers trying to get late clears of old content for the first time, what do you see? "Unsynced, please help me get my first clear," and not "Synced, I want to experience this as intended."
2.) Challenge? Same thing as balance, refer to #1
3.) Newbies getting discouraged by veterans unleashing flashy skills? What? That's the complete opposite of everything I've personally observed in all MMOs I've played since the early 2000s. In just about all MMOs I've played, including FFXIV, when newbies see veterans pulling off cool, endgame attacks and combos, they are more inspired by it rather than discouraged. It was the same in Maplestory when newbies aspired to reach high levels to pull off the cool stuff the pros are doing, and it's also the same in FFXIV, when sprouts thank me or get excited or even try to befriend me when I use my endgame AoE combos and such to one-shot mobs that the sprout didn't mean to aggro and almost overwhelmed them.
Based on my personal experience, if old content is going to be a mindless faceroll anyways, and both veterans and sprouts are just trying to get them over with ASAP for different reasons, then the only thing that letting veterans have their full kits in old content will do is to make old content less boring for them. Removing skill sync won't turn these old content into a faceroll if they're currently already a faceroll to begin with, IMO.
I mean, let's be honest. Just about most people would prefer braindead easy content where they get to use 24 skills as opposed to braindead easy content where they only get to spam two buttons over and over.