If any object is dropped from a great height then until it reaches the absolute bottom it will continue falling due to the gravitational pull. If man is asked to ascend during this descent (kaliyuga), it will not be possible for him, hence he shold be allowed to descend.
But descending gradually when he reaches the abysmal depths, to prevent him from being crushed into smithereens, Lord Krishna conferred to man the mainstay of the Gitalike rope.
If anyone descends taking recourse to a thick rope then the possibility of crashing down is extirpated.