It's doubtful if you know even a basic bit of chemistry. Vulcanized rubber is a crosslinked thermoset polymer that is very difficult to break down, regardless of whether it is natural or synthetic in origin. It's going to produce roughly the same amount of microplastic per amount of tire. Lets say natural rubber lasts 2x as long, that's half as much microplastic, not 1/1000th. I get that GP is being hyperbolic, but natural rubber is incrementally better (at best) than synthetic, not revolutionarily better.