NASCAR used to have a rule and I think it's still in place, that limited the number of race teams a single owner could run. It stops 2 or 3 team owners from taking over the decision making of NASCAR. What if the NBA and the Players Association limited the number of players an agent could represent and still be certified .
It wouldn't solve all of the issues that currently exist the OP describes.
One of the issues I have with the AD/Lakers/Lebron stuff is I don't think Rich Paul is really looking out for ADs interest. This to me as an attempt to solidify LJs legacy more than look out for ADs long term interests and his potential legacy.
Limiting how many clients and agent can work for would minimize potential agent conflicts of interest.