What if anti-lock aim would randomize the angle instead of pinning to 0/90?
Problem solved.
As said above, notchers can just aim straight up and start counting, non-notchers still have to guess every time.
Nope, we still have Circular Aiming, which although doesn't fully prevent it, makes it much harder. Also it is very obvious what people are trying to do when it happens, which should discourage them from doing it.
Changes to the engine to prevent discrete aiming steps are possible and have been considered (so no single tap of an arrow key is the same, and counts fractions of a frame/pixels), as well as programmatic rotation of the crosshair graphic.