Hmm.
So if I were a seminary President, I might think that the end game of what I was doing was training people to lead successful churches and ministries. Right? I mean, there are added things as well…you’re allowing people to deepen there personal theological perspectives and awareness, training people to be the next round of professors, etc. But ultimately, you’re training people to lead successful churches and ministries.
So two things occur to me.
One: why don’t seminaries sponsor a survey of their 10 year out graduates to see how many of them are leading successful ministries? The seminary could define the parameters, but ultimately, that would give the seminary the right to say, “We’re a seminary that’s good at what we do. We’re effective.” You would think that that would be the ultimate test and ultimate bragging rights of a seminary. “Our graduates prove to be prepared and competent.”
Two: why haven’t they? I mean, I can’t imagine that I, sitting here in my armchair, am the first to have the idea. Hasn’t some seminary President thought of this before me? And if they have, why haven’t they proceeded? Which only further presses the question of whether or not seminary is an essential part of ministry preparation.
That said, I’ve sat through enough sermons that were as deep as the inflatable kiddie pool to rule it out altogether.
Hardwired 
I would imagine any college or university could be asking the same questions about their graduates. Is a bachelor’s or master’s degree “essential” to a successful career? Only if the person/organization who might be hiring you thinks it is.
Bill Gates didn’t finish college and Joel Osteen didn’t go to seminary, but this hardly proves that higher education is a waste of time.
What would you measure? What counts as a successful ministry?
That sounds like a GREAT dissertation topic. You seem ready to crank out another one!