The issue isn’t so much left or right foot braking but more one foot v. two feet. As has already been mentioned plenty of people have the use of only the left foot, which is fine because the foot is only going to be pressing one pedal at a time. And that’s the point, those who build the muscle memory of left foot braking and right foot accelerating stand a very good chance, when the chips are down and instinct kicks in, of stamping on the brake with one foot while stamping on the accelerator with the other, often with disastrous consequences. By maintaining the foot off accelerator onto brake instinct (and relearning to keep the clutch foot doing nothing, which is learnt very quickly) you eliminate this, very real, possibility.