If you released super popular whatever (game engine, embedded platform, 3d design software) that uses D as its customization language suddenly a lot more people would have a good reason to learn D.
The largest-scale usage of D that I have personally seen is a AAA game studio who had an in-house 3D-engine in C++, and chose to write plugins and game logic in D because of easy integration and hot-swappability. AFAIK worked out well for them.
