...and if he did it would greatly increase the possibility that he would not have a successful opening of the backpack that he took.
Why do you say that?
The airliner was above an overcast and, since the crew reported rain, there were apparently cloud layers above them. So it was pitch dark when he jumped. Cooper would not have had any references to help him stabilize. He could have pulled the ripcord while at the bottom of the stairs but there is nothing to suggest he did so.
If Cooper delayed pulling the ripcord for even a second after leaving the stairs, he would be tumbling and if he had an asymmetrical body load it would be some really bad tumbling. And if he didn't pull the ripcord, he would be on the ground in somewhere between about 40 seconds and one minute. If he did have an open canopy, then why wasn't it found unless he landed in an open field. If he landed in an open field, it would be a miracle and just how many miracles do you get in one jump?
377 has posted on this very subject. He has jumped on a sunny day with a small radio strapped to one leg. He reports that he had to make adjustments to compensate for the aerodynamics of that small radio.