I've changed my whole attitude towards sidemount based on what you and Bob have said. I'd never tell you that, but there you are. So, I believe that you can change an extremists mind.
My initial post comment on chainging was too brief because I stupidly posted without that part and added it quickly in an edit. What I was really wondering about is what it takes for someone to step back and realize that he or she is an extremist. I know a wonderful married couple who would have seemed poor candidates for connubial bliss. He was a first class slob, and she was OCD. What happened was that each of them had a revelation about their conditions. He worked hard to become reasonably neat, and she has done what she can to curb her OCD problem. What helps is that they both laugh about it.
Similarly, I have recently read autobiogrpahical sketches from people who went from being pretty serious racists to being active opponents of racism. When they were racists, they would enver admit it. They always had a good, solid reason for the beliefs they held. In each case, though, they had something like a blinding light moment on the road to Damascus that made them realize, "Holy crap! I'm a flaming racist!"
I have talked mostly trivia. I am going to give a slightly less trivial example and then hint at why I am not going deeper into this.
I first started technical diving training in an environment that was very precise and, well, extreme. Every little detail had to be done a certain way. I later switched agencies, and my new instructor(s) did not have that attitude toward the smaller details. I eventually was working on becoming a TDI instructor, and I was conducting a training dive in which two TDI instructor trainers were pretending to be students and screwing up in every way they could so that I could deal with their errors. In one case, I corrected a mistake one of them was making, and he seemed really confused. It took several tries for me to get him to do it correctly, and I thought, "Man, he is really overdoing it."
When we got back on the boat, he asked me what was up with that skill. I told him what he was doing wrong, and he asked me why it was wrong. he always did it that way. Everyone he knew always did it that way. My only response, lame as it soundedd even to me, was that I had always been taught that I must always do it the other way. It had been thoroughly drilled into me, but I had no idea why. I tried it his way, and when I saw how much better it worked, I was converted. Someday I will run into my other instructor and ask him why it was so important that we do that skill the other way, because I really am curious. Maybe he will realize the extremity of that position as I did.
Once again, it was not a potentially fatal skill, but I have several "extremist" policies in mind that I believe are indeed potentially fatal. Why don't I call them out publicly? See bullet #3 in my opening post. The attacks will come hard, and I don't want to start a flame war. I hinted at one of them a few years ago in a ScubaBord post and got a private message threatening to take action against me for violating the PADI standard against disparaging another agency. Well, I'm not expecting that to happen, but it shows how serious some people can be.