This is interesting analysis. I would like to see a study like this done for a broad range of popular site and brands.
Fully optimizing the mobile experience on the full range of mobile devices can’t be done with responsive design alone. Investing in additional HTML formats based on device detection makes sense for Google, Amazon, Facebook and others who can justify it with a tangible ROI.
This does not take any of the value away from a responsive design approach. Responsive Design makes a great deal of sense for businesses who can’t justify the effort required to build and maintain multiple sites or experiences.
What’s more, even if you have custom experiences for less capable devices, you should definitely consider making the rich experience responsive for a range of devices capable of delivering this higher level of experience.