Without iterative improvement, the chance of a complex adaption occurring by random chance are much much smaller. I don't know how complex an adaption to become resistant to this substance is, so it's possible it could occur by random chance. But even if so, it would likely still take much longer.
I can't speak about what the adaptation is, so I don't know if the adaptations at the difference levels of concentration are different in kind or degree. If degree, it is more feasible. Certainly the environment is only different in degree. Keep in mind the adaptation to go from 0 to 10x did not happen iteratively - and I may be remembering wrong, but I think that the bacteria stopped at the 10x boundary longer than the others. That may indicate that the subsequent mutations were of degree, not kind. Anyway, I think it would be worth doing this experiment.
