To my mind, the point of `unless` is that is combined well with something that is unlikely but still possible.

  return the_obvious unless something_improbable_holds

  the_most_astonishing

This also align well with the lake of scalability of the number of condition: I doubt that in usual prosaic English its common to use "unless first-condition or second-condition or … ultimate-condition".

Of course it is logically equivalent to `if not`, but it is pragmatically not conveying the same information at human interpretation level.