RFA by year

I think that could be part of the solution. There might be admins who never should have been admins... as well as other editors who could take on more responsibility but who are not interested in adminship. (I've never been an admin, and never wanted to be. I take it as a complement that a few people have asked me to be an admin, and many more just assumed I already was.)

The most interesting insight from looking at the numbers though: the admin attrition rate isn't especially high. It's hovered around 20%, and peaked in 2007 at 28%. If the numbers are correct, then the real problem is that nobody wants to be an admin anymore (or nobody wants to go through RFA).