I don’t think you realize how big of an undertaking what you just asked really is. In fact, there was another topic along the same lines about a week or two ago that suggested reimplementing zones as they were in Classic via a combination of Chromie Time/phasing, yet allowing flight in them.
Go to the 5:00 mark if the video doesn’t start where it should.
Bottom line: they CAN’T use the zones as they existed from Classic-Wrath as many zones are HEAVILY fragmented. At the time, we didn’t have flight in those zones, and they used good old fashioned developer tricks to cut down on the install size. After all, why fully build and design a building if you’re never going to be able to enter it? Why have that same building viewable from every angle if you’re only going to see it from the front? Why build terrain with proper collision detection in an area your character is never going to be able to walk in?
For this to work, they would literally have to take the zones as they have existed since Cata, cross reference it with the Classic versions of those zones, then design brand new phasing for those zones to reflect a pre-Cata world state. I don’t think you understand how much work would need to be done for something that would have minimal benefit that goes against the core design philosophy of Classic: present the game as close to what it was during that original time period as they can.
Yeah, I know: they completely borked that with their greedy BC/Wrath packs. There’s also the fact that taking out LFD as it was when it was first introduced on the tail end of Wrath has greatly angered some folks, but at the same time I myself feel that the random group finder really was the beginning of the end, so that’s more or less a case of damned if you do, damned if you don’t.
Anyway, while I greatly feel the “nO cHaNgEs!” clowns need to be taken down a peg or two at times (I still remember how insane some of them got at the mere suggestion of adding the achievements system to the original release of Classic, as well as transmog), this is an area I would have to agree with them on. At its core, a Classic version of Cata should be presented as it was, and to even suggest Zidormi/phasing for Kalimdor and the Eastern Kingdoms goes totally against the core idea/theme behind the expansion.
Of course, at the same time some of those “no changes” folks firmly think Classic should end with Wrath, so it might actually be hard to predict how some of them might react to Classic Cata 
If they changed ANYTHING in Cata, I would say keep the instances at pre-nerf launch difficulty. Tail end of Wrath is when the “GO GO GO!” chain pull mentality originally started to take hold, especially since the ICC/Crusader dungeons could get you pretty damned geared even if you didn’t raid. People couldn’t comprehend that the start of an expansion is supposed to be a gear reset, but rather than learn how to properly play again, they just cried like children that the game was now “too hard” and got their way like a bunch of spoiled brats. In turn, that led to less emphasis on “sheep this target, sap this one, etc.” and wound up creating a generation of WoW players that never learn how to play their class well at max level, as well as increased the number of players that constantly die to easily avoidable mechanics, players that ignore their interrupt skills, and overall a group of players that’ll instantly blame the healer and/or tank the moment something goes wrong.
It wasn’t that the game was “hard” following Wrath: it was the fact that players became too braindead to think their way past encounters, and the moment they were faced with any sort of challenge, they just wanted to yell at Blizzard to nerf it. I’d honestly like to see how the Classic community handles all the Cata instances if Blizzard releases them in a pre-nerf state.