That may be true, but I think it's moving the goalposts a bit - if we want to talk about Renaissance art, for example, we really do have to consider David and the like. Indeed, I'd argue that Mannerism is in fact a subset of Renaissance art. It's certainly fair to say that the Renaissance represents a movement towards naturalism in art, but I don't think you can really say that it became the be-all-and-end-all - after all, the most popular subject matter was drawn not from nature, but from mythology, be it Christian or Pagan.