A world revamp does not need ot use loads of resources. The major failing of Cata was that in addition to redesigning all zones to work with flying, they also made entirely new zones. A new world revamp does not need to do that.
Secondly, each revamped zone does not need to have a current zones worth fo content, or even detail.
Consider for instance Westfall. Even fully revamped this zone would take probably a fifth of the time and effort an entirely new zone of similar size would take. Combine that with all the adjacent zones like Duskwood and Elwynn and you can look at a set of zones with similarities like one new current zone, with about the same comparative level of detail just over a larger area.
If oyu then consider a set of zones as one large zone for level design purposes then you can easily envision for instance a version of Eastern Kingdoms with 5-6 "zones", each with its own questlines and reputation.
Graphically however a world revamp is almost done already. Darkshore proved that all a zone really needs are a few choice texture updates, a new skybox and a smattering of new updated buildings and mobs to feel almost entirely new.
Even old world assets like the Vanilla Barrow dens were reused in Legion in such a way that I doubt anyone particularly noticed that most of it was over 10 years old.
Most of the world revamp work is already done though, more importantly. We are close to having updated every single old world mob, and for architecture we have Human, Nelf, Gnome and Draenei buildings for Alliance, and Orc, Troll, Tauren and Belf for Horde. Worgen and Goblin meanwhile are new enough that I doubnt anyone would really notice them being reused. They certainly didnt think too much about the reused Goblin stuff in the BfA dungeon.
The criticisms of Cata are almost entirely focused on what that expansion did wrong when revamping, and ignoring all the ways a world revamp is already being worked on in subtle ways.