I think a quick survey of formerly active English Wikipedia administrators, to find out why they left, is the first step to determining what is going wrong and how it can be addresssed.
I think this is an excellent idea.
Hey Foundation! Can you all make something like that happen?
I think this would be good as well. We got some of this data on the Former Contributors Survey Results, but this was not focused entirely on admins.
I'd encourage people who are interested to organize such an effort, perhaps basing it on the other survey. It doesn't have to come from the Foundation.
We can definitely draft up a survey, and use the old one as a template so that it's easy to compare numbers. But actually contacting the users and compiling the data, we'd need a little bit of help.