You're confusing complete with specific. The material design language offers specific solutions to specific cases. But what designers need more is a general theory they can apply to any situation. And the theory behind material design doesn't fundamentally differ from flat design, other than it's focus on animations. It still shares the principles espoused by flat design people (authentically digital interfaces, minimalism).
No, I'm not, I'm saying design languages with well-defined motivating principles like Material Design and Microsoft's ex-Metro language are both more complete schools of thought on design (starting with identified principles that cover broader scope of design problems) and more specific, well-defined approaches to design/interaction (clearly defining both foundational principles and the applications of those to specific areas of design) than things like "flat" design.
