The few longitudinal studies that have been done are pessimistic -- backsliding on accessibility beginning just a few months after an intervention or campaign is the norm, not the exception. New technologies are introduced to sites, breaking their accessibility.
The 508 Strategy should include an ongonig evaluation and reporting scheme. Being reviewed every 2 years by DoJ is not going to work, especially when the survey changes every time and 2 years becomes 10.
Once reasonable metrics are agreed upon, why not have a dashboard that can be read by all, indicating where agencies are at? There may be even more value in tracking technologies -- "accessible PDFs are up 8%; captioned videos are down 5%" -- because aside from agency pride, everyone wants to know what to use and what to avoid.