Seatbelts were gaining adoption pretty damn fast before they got mandated. Ditto for safety glass, brake lights and airbags. I think if we didn't mandate them the only cars without them these days would be those ultra basic trims that they only make five of so they can advertise a low "starting at" number.

ABS and traction control are the questionable ones that saw their adoption plateau and probably wouldn't have the adoption they do without government. Modern collision avoidance stuff appears to be making its way through the market lightening quick without mandate.