If I'm wrong, someone please tell me. It seems the problem is not only the agency's (State) operations, it's the people who are employed by State. Do they have the experience but aren't volunteering for foreign service? That's kind of what I'm getting from reading much of this.
if an insufficient number volunteers
Not to be snarky, but if you got hired as a FSO, where do you expect to work? From home?
It just appears half, or perhaps more, of the problem is that the people who are needed most just aren't volunterring to go.

Like I said, if I'm wrong or completely misunderstanding this, tell me.