> if it doesn't hurt then you're not making optimal development

this is almost certainly wrong - 100% balls to wall training will surely be suboptimal (on avg) to achieving most fitness goals - eg within a running training block there will generally be recovery and "general aerobic" runs which are easy in effort relative to the harder work in the block. These easy efforts are necessary to optimally achieve the desired physiological adaptations acquired through increased volume and "nailing" the hard workouts. The easier runs enable this by getting volume at lower risk of injury + conserving energy/will for the key workouts.

This also doesn't consider how important recovery is to optimal results (as in sleep, rest, self-care etc).