I have no idea on the statistics, but I hate to think how many guests book ADRs and never show or cancel. We even read on the Dis how some will make multiple ADRs for the same group at the same time, differnet restaurants/parks.
Not to mention WDW is different than most restaurants because so many guests rely on Disney transportation.
I think what Disney has works well.
I cannot count how mant PS/ADRs I have made (and done) over the years. I think the only place we waited past our time more than a few minutes was Rose & Crown, which was a complete bust of a meal anyway.