Those are some interesting questions from dsugar. As a beginner in this business, I look forward to seeing answers to these as well.
There's no easy answer. Take Assurant for example; a standard contract for a writing agent is 20% and a typical GA will stroke 5% for an override. That 5% override is justified since a lot of training and support is given.

But what's an agent supposed to do when they no longer need any training or support? If they do $10,000 a week they're giving $500 a week to the general agent. I can think of a lot of ways to spend $500 extra dollars per week.

And there's the problem. The agents who no longer need their hands held natually say "I don't even need to call my GA anymore. Why don't I just become a GA and get the extra money?"

The only thing a GA can do to keep an agent under them who no longer needs support is to cut them a check for extra money.