It's more that they should do a content squish, which they partly did. Meaning tbc, northrend and cata(highlvl) are on the same tier of itemlvl and MoP and WoD are on same itemlvl.
This basically mean Legion = Wrath release itemlvl.
But with new tech their using in legion. It will be better if old the old content scales on your lvl and make all the old content scale up and hopefully add a difficulty mode that ads special rewards which you can buy high itemlvl gear for. (instead of grinding dungeons which is still more efficient you can get some through questzones).
In a way they could also reduce the lvl of you char as their is much a space where a lvl means nothing. But some people will be negative about that, so your better of doing it indirectly by making making itemlvl difference between old expansions extremely low.
But it's really the multiple raiding difficulties and Warforged element that is driving the itemlvl and thus stats up a lott.