I listened to that podcast last week. I agree that the 450 is probably a bit too much for SX, but the injuries are not only displacement/4-stroke dependent. A big contributor is the track style. Tracks are not built to slow things down at all. They are built with very fast rhythm lanes, with jump faces that allow something like the Tampa quad to develop. Go back in time, SX tracks were much more peaky, rutty, irregular, and slow. Vuillimen has pointed this out many times on various podcasts.
Slow the tracks down, make the riders have more small mistakes and we will see less big crashes and hopefully less injuries.
Old MXer turned Superfan.