The problem is that there is no smooth transition: there is a "valley of failure" in the middle of it, where there is enough automation that the human can disengage from moment, but not good enough automation to avoid all errors, so the human operator has to step in to correct errors quickly, and doing do "cold" (i.e. from a disengaged state) is not good enough.

Dekker touches on this in "The Field Guide to Understanding 'Human Error'"