You're acting as though it's not currently like that. They all have the same 3d face model right now... The only thing that has ever gotten changed is the face's texture. Then you have extras, such as troll tusks/belf ear jewelry/things like that, which I'm sure will still be an additional option with the new models. We're not losing anything, we're only gaining in model quality. So why try to make it more than it is?
I'll admit the panda faces all look similar, but again, you can only texture furry cheeks in so many different ways, the panda pattern was the main thing. If you look at the Blood Elf face textures by comparison, there's actually a large amount of variation with the eyes, mouth, shading in general... which leads to more distinct looks.
With how much the texturing seems to be improving in detail along with the new model revamp, I'm certain once they finally release a selection of faces, we'll see plenty of variety.
And like others have said, these previews are just to show off the base model quality improvements... They're only showing one face, one hair color (oh god they're all going to only be brunettes, the sky is falling!) and etc.