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Actually, it's COMPLETELY valid.
Your injury and bike damage per mile argument, while factually correct, doesn't actually have anything at all to do with the environment itself in which the riders are riding. Instead it has to do with how the riders themselves are riding in the environment that they're in.
In a closed course environment the likelihood of outside forces (other traffic, surface debris, etc) being a contributing factor to a crash are virtually NIL. Zip, zero, nada. Thus, we ride much harder & closer to our limits for pretty much the entire time we're on the bike. You don't do that on the street, do you? Of course not.
If we all rode on the track like we do on the street, I guaranfuckintee you there would be LESS injuries and bike damage per mile at the track compared to the street and your argument would be proved invalid.
If you flip the scenario and we all rode on the street like we do on the track, I guaranfukintee you there would be MORE injuries and bike damage per mile on the street compared to the track, and again, your argument would be proved invalid.
It's a proved human phenomenon.... if you reduce the external risk (environmental, equipment, etc) we simply increase our own internal (behavioral) risk. In addition to that, it could be argued that those riders who take it to the track assume more risk in their riding to begin with...
All of this is a long way of saying this: The TRACK presents no danger whatsoever to the rider. It's the rider that presents the danger to themselves.