There is no proof, source, or even reliability to this.
I think it's safe to say this is one of the few pieces that we're not talking about a "vocal minority" about.
YES, you're right, some players might show old models if given the chance (and I believe they said they'd want to accommodate those kinds of players when making this change), but don't get ahead of yourself.
WHEN models change, and IF there is option to show old models VS new, I promise you, I'll even bet you my car (2008 400h), that the greater (and I mean VAST) majority of players would not enable the option to show old models.
Yes, players hate change, but there are two problems with that kind of concept.
1) Each and every single facebook update ever.
Lets be honest, how many friends have quite facebook and I mean *QUIT* facebook over an update.... people love to bitch and moan, so they do, and then they forget, why? Because most of the time they realize what they were bitching and moaning about in the first place actually was quite great! Most people on facebook can't remember what the layout looked like right before this last one, let alone 6 months ago when they claimed they were quitting facebook if they didn't change back.
Sure, people complain, but people complain about everything, but it's a visual upgrade.
2) They hate change when you actually change the mechanics of things. It's not like you now need to learn how to QWOP your character with the new models, it's a visual upgrade, (mind you, as Blizzard has stated, this has been one of their most requested features of all time). People who will want the old models will not want it because they hate change, but because of nostalgia, or because they legitimately like it.
They're not changing how the game works, how you run your character and how management of your character changes, just how they look. Yes, nostalgia will get some people, but it's still the SAME EXACT GAME just looking better.
People want this change, and when it does happen, the actual vocal minority (complainers) will forget about it in two weeks time and laugh looking at Wowhead or Mmo-champion archives at how models used to look and say "hehehehehe I can't believe the game looked like this!"
Just think about the druid forms.