A normal expansion has 4-5 zones on average, at the start, with, on average, two more added after
There are 22 zones in Kalimdor, and 25 zones in Eastern Kingdoms as a baseline. 47 old world zones. If you want to drop Teldrassil and the two that got revamps, that's still 44 zones. This isn't including any new requests for revamping blank spaces. That's as an absolute baseline.
Simply looking at math it'd take more like 10 times the workload. And we know from Cataclysm development they infamously underestimated how long that'd take, and that was off the tail end of WotLK when WoW was as popular as it ever was, so we can't reliably say it'd take any less. Sure, they don't have to make the entire land from scratch, but they have to redo the entire questing experience. For something that, let's be honest, 90% of the player base won't engage with if it isn't the quickest path to levelling.
I remain very much against a world revamp and very much wanting selective updates (Azuremyst/Bloodmist and Quel'thalas being linked to mainland and getting flying, plus updates).