> as many 9s to the likelihood of success as possible
If that were the actual mission of the FAA, that would be easy to accomplish with near-infinite 9s: Stop flying entirely, ban all flying machines of all types. You'd have zero aviation incidents if there was no aviation.
There are a finite number of 9s that are needed before the cost of adding more safety is detrimental to humanity.
And note that I do work in a field where my work could kill someone if it messes up. I build industrial equipment, typically to ISO 13849 PLd or PLe standards - that's 7 nines of probability that there will be no dangerous failure in any given hour, which requires reliable components, fault sensing/fault tolerant safety controllers, and redundant wiring .
I could make equipment I produce more safe, if that was my only goal. For example, each E-stop button has two contacts and two pairs of wires in parallel which monitor its state, and this button removes power from the machine by turning off two contactors in series. If any one of these components should fail - a contact on the E-stop gets shorted out, or a contactor gets welded closed - the system will detect this and remove energy from the system. I could, in theory, make this "more safe" with three pairs of wires and three contactors in series. But while I've had to fight hard against those who try to reduce the safety level in favor of production efficiency, I've never tried to add a third set of redundant contacts.
In the end, an operator standing next to the E-stop button and supplying one of my robots outside the safety gate and light curtain means that there are 6 guys who are not using high-power hand tools to do what the robot is doing. Adding excessive 9s to this system means not that people are safer, instead there's less automation and the world is worse off.
A simple registration process, knowledge test, and inexpensive transponders on RC aircraft would make the world a better place. Requiring hundreds of hours of training and certification, and adding prohibitively expensive transponders, would kill off RC aviation. I want you to fly in a plane safely. That's harder to do if your pilot didn't grow up at the model airfield.