What's wrong with this design? Pressure pushes the plug out of the cabin instead of holding it in place. There can be 20,000 lbs of force on the plug depending on the inside/outside pressure differential. (psi * length in inches * width in inches).
Looks like Boeing did a cheat and the door plug has a slide up and down feature to get specific bits of the door on the inside of specific bits of the plane. But the door plug must have slid some, at 16,000 feet.
Boeing engineering used to be belt-and-suspenders operation, but MBAs correctly computed suspenders cost money.