That's what all substats and defense already do. They don't really "shink" but reset as the strength of X substat is dependent on level.
Does this also mean that when we sync down to old content like Rabanastre it won't be as much as a roflstomp? Would be nice to see some mechanics that you never see anymore.
Syncked duties that also iLv synck you will probably feel the same, because your relative power level now will be about the same as your relative power level post-squish. Strictly speaking we will be slightly less overpowered across all the cap level content, but it probably won't be noticeable except at the lower end of each level cap.
For example, Labyrinth of the Ancients, Void Ark, and Rabanastre should be harder than they are now, to some degree; while World of Darkness, Dun Scaith, and Monastery, will be about the same as they are now.
But again, this all depends on exactly what the new curve will look like, so we won't know the real impact until Fall.
This is absolutely something they could control, and I think they should look into.
Some potential places to reduce the explosion of item level:
Leading up to the end of the expansion, we were given a job coffer with item level 430 gear
Immediately we had the option to replace this with item level 440 Ronkan gear
Then level 445 crafted gear
Level 450 Eden normal
Level 460 Tomestone (and later 460 YorHa gear)
Level 470 Eden savage (and later 470 augmented tomestone)
So within the first patch we went up 30 item levels.
An example of the squish they could use:
Coffer : 430
Ronkan gear : still 430, but lets you use the same items on multiple jobs to streamline armory space. Additionally this set would have the accessories. Alternately this set could just be deleted.
Eden normal gear : 435
Crafted gear : also 435
Tomestone gear : 440
YorHa : 445
Augmented tomestone & Eden savage : 450
This cuts out 20 item levels of advancement every tier leading to 60 fewer levels added each expansion.
The stat gains could be comprable as well, which would lead in addition to the stats not inflating so dramatically, more flexibility when creating BiS lists for jobs making 24-man raid gear more relevant than it currently is.
No titles or loot for unsynched runs, sounds like a good plan to me.
iLv adjustments are not necessary, and the iLv number itself is largely meaningless outside of its role in locking people out of content. The iLv growth scheme we have allows for a certain perception of the expected pacing of gear replacement, and is fine. They can easily adjust the gear stat formulas without changing the iLv scheme.
It also heavily depends on the job changes. While ilvl does make the old content easier the biggest reason why alliance raids like LotA, Void Ark, etc are such an easy snoozefest now is because of the potency changes SE has made over the years to compensate for removed mechanics or because they were needed to balance jobs at max level. Overall our character's abilities simply deal more damage at level 50 now than they did in ARR.
I'm saying that actual iLv values are irrelevant to the problem you describe. They just need to set the equipment stat formulas so that the stats on the gear scale appropriately. This can be done without changing the iLv of any equipment we have, and without deviating from the current 130 iLv per Lv cap scheme.