Luke has answered this elsewhere. It's unfortunate his positions on specific topics aren't indexed somewhere that he can just reference by code. 

His position is that from inception the field was standardized at 40 bytes, intentionally to limit spam. So reliance on and design around an expanded field size is a mistake on their end, for non-conforming to the "spec". 

Things like this in protocols tend to be worked out by representative bodies and industry boards such as w3c, iso, among others over time. These standards also tend to compete for adoption.

Bitcoin being money, a new project and a philosophical or religious paradigm shift for participants, with enourmous social consequences, it will take a while to smooth out the standard.