Could you clarify "can't do that"? I've been driving my new Model Y for only a few weeks and have no experience with other models, so perhaps I'm missing what you're saying. The regenerative braking absolutely enables one-pedal driving in most situations, but not in unplanned hard-braking situations where the stopping power of the regenerative braking (at least as implemented in the firmware) is insufficient by itself. If you plan ahead and become good at feathering the accelerator just right, you can mostly avoid using the friction brakes (i.e. the brake pedal). Admittedly however, sometimes this would result in a stopping trajectory that may be a bit too slow and may annoy drivers behind you even if you are happy with it.
This last comment leads to a point that I've thought about several times (and may be what you are getting at too), which is that the retarding action that we call "regenerative braking" could easily be supplemented by deliberate motor-reversal torque. I don't see any reason why the motors can't operate with as much reverse torque as they do forward torque (but then I didn't design the motor system). This kind of enhanced motor-braking would not be entirely regenerative; it would benefit somewhat from regeneration but would also need to draw some net battery capacity to increase the effect of motor-reversal braking.
So to me, the interesting design change would be to intelligently couple the brake-pedal control to implement:
a) motor-reversal action for primary braking beyond regeneration alone,
b) conventional disc friction brakes only for secondary hard braking (emergency and safety-backup),
c) friction brakes for holding the car after the above a) and b) methods have brought it to a stop.
Method a) would greatly reduce the wear on the friction-brake pads (though said wear is already less in an EV than in a conventional car), and continue to give some measure of regenerative benefit even in hard stops. I don't really know for sure they don't already do this, but I haven't ever seen it explained as such. Method b) involves a long-understood fundamental safety principle, that cars need to be able to stop faster than they can accelerate for a few very good reasons. Method c) is there to allow essentially zero energy use in keeping the car still, and also creates almost no pad wear in use.
Pertaining to your FSD example, the car in autonomous mode need not use the pedals at all. All of the above braking modes could be deployed as needed by the FSD computer, mostly modes a) and c), again with b) reserved for unusually hard or emergency stopping only.
Interestingly also, the car could he programmed to perform a brake-validation self-test procedure at startup and/or occasionally while stopped: apply motor torque very briefly and verify that the wheels are not moving by even one click of the sensors. If they do move, that's an indication of a friction brake degradation or failure, and the motor control can be used as a brake substitute, presumably with a limited operational envelope until the friction brakes are properly repaired.
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