I don't understand why the plug doesn't use pressure as a failsafe. You have high pressure one side, low the other side, and just like normal airline doors the interior pressure makes it impossible to open the door.
The more it pressurizes, the greater the force jamming the door shut.
However, since it's a door, it has to be able to open. The door is constructed such that if you slide it upwards, the ears clear each other and the door can come out.
There are bolts to physically block the door plug from ever sliding upwards. If the bolts were correctly installed, or ever installed at all, the door would be perfectly safe.
Ultimately it's extremely difficult to make a plane where you can just not install a bunch of bolts and everything still keeps working fine. You have to have a high quality manufacturing operation to make a reliable plane.