GM director of design, Ed Welburn, has conceded that they will definitely not be using rear fender skirts because of many production, cost & marketing drawbacks, even if they do reduce drag & improve fuel economy.
Clearly for the OEM industry, people just don't like them on their designs.
The aftermarket industry and fellow eco-modders can & will fill that 'void'.
Rear fender skirts look great, but flunk in other ways