I feel like from this bit, you're missing the most vital reason for updating the visuals of the old races.
The complaint was never really solely concerning the raw model/texture/basic appearance - the important thing is the appearance/silhouette viewing the character from behind and the way it looks when it does things.
The prevailing thought (I would hope) is that most people spend a lot of time spamming this spell or that, watching auto-attack swings and seeing your character run. All of those things need updating because it's a hell of a lot more satisfying to watch the newer races "do things" than it is to see the older ones use the same four animations for several hours a week.
So, if you were considering countering me by posing the question "would new skeletons and old models have been enough?"
The answer is yes. The skeleton/rig is the most important part - the poly count and texture doesn't matter as much - those things don't give as much "life" to the characters as the skeleton does and it doesn't matter from what angle you see your character, you still see what they're doing and how.
Do all things "get old" over time? Yes. Do Pandaren and the like have more amusing animation sets and even more white attack animations? Yes and they're better off for it.
Tl;dr - The big sell for updating the visuals is that the skeletons provide an enormous injection of "life" into the player characters and I think a lot of people value that - whether or not they appreciate the model/texture updates at all or for very long.