What holds back refinement of self-energizing bicycle brake designs? Existing examples, that I've come across at least, are generally a mess and ride more like early proof of concept experiments than something that should ever have been put in the hands of normal people. They're difficult to modulate and add what feels like an arbitrary and unpredictable amount of power to the braking system. This impression of them became more or less entrenched, and that's essentially the last we've seen of them.

But why did it end there so decisively?

In the canti design, instead of completely replacing the pivot with a helical interface, why can't you combine the two? I.e. have a traditional pivot doing most of the work but with a helical interface with a very short, controlled amount of travel layered alongside it to amplify the power up to a certain point. Or, similarly, in a disk brake, couldn't you have traditional pistons but at the end of them there's a little track angled on either side towards the rotor for the pad to get pushed down, similarly adding a controlled amount of braking power?
Power assisted brakes that amplify the amount of force applied to a bicycle wheel hasn't taken off because the idea doesn't provide a large enough a benefit of increased stopping power and saved energy required to stop to counter the cost of R&amp;D.  

Cars have mechanical assistance because of the amount of force that is needed to bring that much mass to a stop is not possible with the amount of leverage and force a human can safely do alone.  To get the vehicle to stop in a safe distance, you need to exert a huge amount of force onto the wheels to get the car to stop.

Bikes with their human passengers require much less force to stop.  Stopping power is sufficiently amplified through mechanical leverage and friction materials - enough to easily lock up a front or rear wheel with a slightly tighter squeeze of your hand.

My impression of an enhanced brake system on a bicycle is that your range of power application would be narrow due to the limited range of motion in your brake lever. Your brake lever already moves so slightly from no-braking to full lock that a device that amplifies the amount of power applied throughout that range would send your brakes from 0 to 11 near instantly.  Of course you could tweak the device to be more sensitive - or - just use mechanical brakes and not worry about a more complicated system.