I don't particularly like the "2 piece rim" you describe, but I have never known of one to come off. I have a couple of 3/4 tons that have them and I don't worry about them. I like the 3 piece better. It has a solid lock ring and a split snap ring that holds it in place. Yes, air pressure holds any multiple piece rim together. It is up to the person who changes the tire to see that it is seated right. We have had an inflating cage in our shop for over 40 years. We put just enough air in them to seat the ring while it is in the cage so we can watch it. We then clip an inflater to the stem that is set at the desired psi and then stand back. We switched to tubeless tires on our over the road trucks back in the early 70's. Not out of fear of the multi piece rims, but because almost half of our flats were caused by folds in tubes. Also, many fires were caused by friction of the tube and flap inside of a flat tire. An unnoticed flat on a tubeless tire does not generate that kind of heat. I run tube type tires on all of my old jalopies and don't worry about them. I would never attempt to use a Firestone RH5 wheel. They were dangerous to the point that when we found one, we took a cutting torch to it so that no unsuspecting person would ever use it.