The rationalizations always put the issue back on the departing employee, "They were burned out", "They had lost their ability to be effective", "They have moved on", "They just haven't grown with the company" never was it "We failed this person, what are we going to do differently?"
Now an individual manager might do some introspection, but even then the change impact of the departure is insignificant compared to the change that can be wrought by being there and honing your persuasion skills.
And given that, the exit interview is a nop.
It honestly probably wouldn't have been as bad as it was if management hadn't tried to coddle the leftovers by telling them everything was going to be okay. Their acknowledgement of the loss made it apparent they were scrambling to figure something out.
When I left Sun I got an email from another long time Sun employee who had just quit as well. They said that if I was leaving then the place must be irredeemable and so they were moving on as well. It left me feeling profoundly conflicted because while I had given up on the company by that point I had never intended to cause others to leave.
So I have to accept your point that a company can be changed by the departure of an individual. It may or may not be the change the individual was seeking however.
One of the most interesting type of personality is the useless guy who brighten the office up, once they leave, the whole office can fall apart, especially if the office is full of strong personalities.
I highly doubt it unless the employee was a C level. Even then, people come and go at companies. I have quit enough times to know that truth. Just as an example, I call up my old bosses or colleagues and I hear "Its all the same here. same shit different day"
The point is that if you do want to bring change which is extremely tough (even for obama :),you need to stick around and be the champion of change.