From what I've mostly heard, wow runs off the wc3 engine which is nearly 12+ years old as of now. Of course with technical improvements added to it WoW wouldn't be as good as it is now but this got me thinking, when MoP was during beta there was a screenshot of a panda with high res textures wearing old armor from vanilla wow and it looked really bad comparing the detail of the panda and the armor being worn.
Now imagine new models come out with new animation and everything and you're wearing that old armor from vanilla, it would look awkward isn't it? Not just that but some areas in the world actually like elwynn or duskwood although they were refurbished when cata came out but some areas still look old for the new models being made and besides WoW is nearly 10 next year so wouldn't you guys think there should be a new graphical engine if not at blizzcon then within the next 1-2 years?