First off, while I am not going into specifics because I am not looking at getting banned again, you could look around to see what players without dev tools have managed to create using WoW assets. It is very impressive.
Second, there absolutely are ideas whose scope is insane and I agree with you on that. But there are also ideas that are entirely possible. The following seem to be the scenarios most often discussed.
The full revamp idea in which the entire world is remade from scratch and on a larger scale at that is just a pipe dream. Oh they absolutely could create a world model like that (see above) but populating it with quests, events and NPCs would take far more time. It's the equivalent of making 4+ expansions at once.
But
The partial revamp idea in which instead of adding a new set of 4-5 zones they revamp one of the subcontinents of Azeroth is no more work than an expansion. The six obvious subcontinents of Azeroth (North, Central, South Kalimdor, Lordaeron, Khaz Modan, Azeroth) are all somewhat similar in area to Dragon Isles (and likely smaller than Dragon Isles will be when we account for all post-launch zones). The main issue ofc is that having anywhere from three to six expansions devoted to revamping the world could prove very boring. On the issue of scaling, zone borders can be changed to create bigger zones so rescaling would not be necessary. Most old world zones are singular biomes; modern zones tend to have three on average.
Then ofc outside revamp we have ideas for a visual update. That is very much possible. We've talked about it, most world objects can be seamlessly replaced without disturbing existing quests and mobs. There would still be a lot of work to do, especially with things like the Quel'thalas/Lordaeron border as well as many coastal areas that don't need to be populated but still would need to be finished to modern standard. The issue with a visual update ofc is that unless it comes together with a partial revamp, it doesn't have relevant content
Another thing is the idea that if they are planning a revamp, they have been working at it for a while. I am not really sure about that. Yes they have created many world objects but the evidence for a new world model is not there as far as I know. They have gotten better at hiding things but that's a fairly recent thing.
I still think we are getting Avaloren (and hope they actually stretch it out into a two-expansion arc and make it huge) but I don't think that all of the scenarios above are impossible. Yes, a full revamp probably is. But not the other options.