You fundamentally miss the point. This is a Defensive Driving tool for bikers. Drivers are the primary threat to non-drivers on a street, and this is intended to reduce risks that drivers in 4,000-lb+ metal cages coming double or triple your speed behind you pose if, as the person in front, you can have any control.
Although, I would concede that extra alerts probably won't save a biker when a driver intending to do harm or even not paying attention (because of their phone??) And drives up fast behind on a collision path.
Trying to think of a car analogy equivalent.... itd be you're in a car or truck and 80,000-lb dump trucks 20x your weight) outnumbered you on the road by 10x, and they could go 0-60 in 8-secs and go 3-4 times your speed by pressing on a pedal. You might want defensive, rear facing alerts more than just your mirrors.