Problem is that you're not used to applying pressure with your left foot which can lead to unexpectedly braking too hard. Also, if you're hovering the brake pedal with your left foot you may actually be resting on it and applying the brakes without noticing.
I've also seen the argument that during a crash you smash your left foot instinctively to the floor to brace for an impact. If you're left foot braking (or hovering over it) you may, unintentionally, press both the gas with the right foot and brakes with the left foot as you brace for an impact.
I drive for a living and I don't see any benefits that would be worth "relearning" to brake. I doubt left foot braking would save me from a crash because I'd be saving what, half a second? Not to mention it would feel awkward as hell. Probably spend too much time consciously thinking about it and it may become a distraction.
What you may want to try is kind of resting your foot half on the gas pedal at an angle. This is mainly for when using cruise control. I'll try to explain what I do as best as I can. By the way, I transport patients and it's 80% interstate driving around Illinois, but mainly Chicago area.
The arch of my foot will be on the lower part of the gas pedal at an angle so the right edge of my foot is sort of bladed/rolled on it while my toes and upper foot are hovering over the bottom/side of the brake pedal. This does three things for me. 1. Allows me to accelerate if need be without much effort. 2. Allows me to brake slightly. If a hard brake is needed, I simply apply the brake and begin to move/slide the rest of my foot. 3. Lets you move your foot around a bit to alleviate pain or prevent it from falling asleep. If none of that makes sense I might have to snap a picture haha.