So glad you are promoting left foot braking, it's been my method for decades. I have used a range of manual change and automatic cars, and I have no problems with using my left foot for the clutch in a Manual and for the brake in an Automatic.

Not only does it improve safety by allowing the car to be slipped very gradually up to a junction but it also allows smoother application of power or brake so we reduce the potential for lurching seen when drivers abruptly switch their right foot between pedals. Bus drivers are especially in need of learning this technique.

I recently drove the all-electric two pedal Tesla. It's incredibly smooth and so doesn't much need any assistance from this technique but it's still sensible to hover a spare foot over the brake ready for immediate action when conditions are difficult. It's amazing how slow people are at understanding the simplest things, but keep plugging away with this one, you are absolutely right.