I'm currently satisfied by the artstyle shown in the concept art, but I think that the OP has a valid criticism on the "plastic" look in the early screenshots. Still, I feel that this flaw can be fixed through a change in lighting and the shininess of the shaders, rather than a shift in overall artstyle, as the OP suggests. For example, observe this screenshot of caverns from early Yooka-Laylee development:
And this screenshot of the recent Jungle World at night:
Lighting makes a huge difference, doesn't it? The environments look less manufactured and more handcrafted, when it gets darker, since there isn't as much light reflected off of the models. Since the shaders' reflectivity was reduced, the environments look more like clay, or paper. Given that the game takes place in the context of storybooks, I can understand why the artists would go in this direction. Just have patience, and I'm sure Yooka-Laylee can be a work that stands on its own from Banjo-Kazooie, despite being inspired by it. That's what a spiritual successor should be, after all.