I would like to make a case for moving more browsers into the grade C category.
Yes please. As a project that must live the test of time I think we should be focusing our energy on building for future browsers. Our main goal is to provide people knowledge which can be done without JavaScript. Older browsers hold us back in my opinion.
Personally I would throw IE8 and below into the grade C category. We can maintain good non-js experiences in these browsers and save a lot of development time on JavaScript debugging.