Gearbox serves more purpose than just getting the power to the wheel, it also improves the efficiency of the motor and\or expands and optimizes the usable speed

Gearbox serves more purpose than just getting the power to the wheel, it also improves the efficiency of the motor and\or expands and optimizes the usable speed range of the motor. Also cars can stop way harder than any regen can, massive amounts of energy will be at play and likely any solution is more complex than just grabbing a rotor, which is a more direct way to heat dump