TBC could use art assets from WoD.I agree the old world is obsolete in terms of quest design and being tied to Cataclysm. But there will always be a part of WoW that is "old". Do we revamp TBC and Icecrown? Where's the line between revamping these old zones but not those? Would there be any value in revamping TBC. If the issue is just moving Azeroth past the events of Cataclysm and allowing TBC and Icecrown to exist in their own little bubbles detached from the "main" timeline, then there is an argument to be made there.
Let's break it down by Expansion
Vanilla - Needs revamp due to events of Cataclysm and general zone and quest upgrades
TBC - Could use an upgrade on a few quest lines but overall the experience is not as broken to warrant a full overhaul
WotLK - Same as TBC except quests were a lot better organized here than in TBC.
Cata - Would get revamped with Vanilla Zones (except for Cata only zones like Vashir (sp)
MoP through Shadowlands - Do not need to be touched.