That is not what I said. Given the proper circumstances and the talent of the average person to screw something up, no matter how well you engineer "one way assembly only", someone will find a way to put a square peg in a round hole. I see this at times in my day job. Usually proceeded by "here's a hammer" try that.
Would it be the ideal situation where it could not be messed up? Yes. But as long as "some assembly required" is on the box it just isn't going to happen. While it is tragic that people die from their mistakes, it is also a fact of life. The only way to completely avoid that is to not live in the first place.
Rebreathers are complicated yet simple pieces of equipment when it comes down to it. I've only been on one and that was just a rebreather experience. One where we completely disassembled the rig, followed a tear down check list, reassembled it according to the assembly checklist, tested it, and got in the water for a planned one hour or so dive. Mine lasted 20 minutes and I had to bail out to my OC slung 40. Even following the list right after entering the water and descending the mouthpiece split on the side and every breath started filling one of the counter lungs. Had I been just a little more aggressive twisting and pulling on some things like the mouthpiece I may have caught that and caused the failure on the surface.
I keep that mouthpiece and use it as a reminder when it comes to the importance of following procedures and not being tentative. Such a small thing that is now on my assembly checklist for OC. Just in case.
You can't engineer total safety. Even if you made a unit that was sealed and could only be opened and closed at the factory. It will sometime fail, the user will fail to operate it correctly, or any number of other things.
Diving can kill. Plain and simple. Personally I make that crystal clear to every student. We all should. But more importantly we, as instructors, need to remember that ourselves. There are a number of dead instructors who seem to have forgotten that and got in over their heads. They paid for it, dearly.