It's not just we don't have old skills, but there are also relatively new substats and damage potencies on top of the sync. When everyone is swinging with potencies that are much higher, old fights scaled around lower potencies aren't going to last.
Factor in that when we sync to content that does not have a direct iLvl sync, only a level sync, then it means that everyone with gear that's over the max ilvl of that level is synced down to 5 iLvls over what was actually possible at the time. This is done to match the best weapon at the time. Even at the time, if you went and did Halicarnassus normal with a group of i400s with i405 weps, she would disintegrate just like she does now.
So what you are asking for, is for the dev team to make separate iLvl tiers on a patch by patch basis, rather than an expansion wide one. That means adding actual iLvl restriction to each tier such that for Sigmascape people would only get synced to i375 Deltascape i345 etc I don't think that's too big of a thing to ask, but it's not really in demand.
For your pros
It would be nice if newbies could be shown the whole fight, but there's people skipping a lot of stuff outright with jump potions anyway.
Newbies won't be prepared for relevant content regardless of how "difficult" the old content is made. Especially if they skipped it with jump pot.
There is no incentive to increase skill, since you can be carried through the normal mode content by the teams you queue in with, so long as they win.
The problem of cheesing entry for roulettes has less to do with how we sync and more to do with minimum iLvl reqs. It would still happen.
As for cons
Slower won't be more fun, like even if the devs could find some way to sync every duty down so that we had to do all mechanics, there's just a lot of folk who would skip out on them entirely and use Fates only, as many already do.
The only crystal tower raid that required two tanks was Labyrinth of the Ancients, and that was due to them not having any data on how the players would do 24 man. All that would change if we unnerfed Bone Dragon skeleton damage is that the two OTs would pick up a skeleton each, and it'd be left to the DPS to get the remaining 4 and kill them far enough away from the other skeles as to not wipe. Nothing really needs two tank parties.
Last con I already touched on. You gotta consider the wild potency increase on top of the sync. They aren't ever going to implement a, "And now your 200 potency combo opener is back to 150, and your 500 potency combo finisher is now back to 340!" sort of sync. And they probably won't revert Direct Hit to Accuracy or Tenacity to Parry. No matter how much they restrict iLvl, for the given content, our damage floor is only higher.
I'm just giving you my two cents. I heard a stat squish is happening in 6.0, and this may make old stuff harder... maybe.