Soggy, if you note they are not my rules, I just relayed them as I remember them. Certainly following them entirely eliminates the argument for all this air redundancy--lol--well--maybe not. Yes diving has evolved, rebreathers are taking people to crazy depths and apparently some people purposely deco dive with no chamber and no surface support.
I am not all that old, I took my first class before I could be certified as a child, took it a second time for cert, they had silly rules back then too. I don't think rule number 1 is all that outdated for SPORT diving in OW. In my prime I could easily or not so easily free dive to 60 and 70 plus depths so twice that put my dive limit right at 130 feet which is considered the sport diving limit even today. I have known many people who could free dive 40 to 60 feet and some much deeper. The idea obviously is that in lieu of redundancy should an air problem develop you should be able to make a free swimming ascent. Obviously, again, things have evoloved like the modern divers need/dependency on equipment such as pony bottles and what not and OK, so that is good you say but then is not staying within a depth range to whcih one could make a free ascent not also a form of redundancy?
"Man, I thought I had been diving for a while! (I started in 72, when we at least had single hose regulators!"
LOL, if Bill Main or some other noted individual told folks that double hose regs were the way to go they would be out scouring ebay lickety split!!!!!!!!!!! Since you have been around a while also then you have probably noticed the re-emergence of back inflation BC--wings as THE system of choice for example so all I am saying is that don't be surprised if double hose regs make a comeback! Cousteu used them to the end of his career and he certainly knew a thing or two.
A modern designed servo/pilot assisted double hose with HP and LP ports for using spgs and mounting a single hose second for octa and inflators would probably out perform any current regulator on the market. AL still makes a dh reg for the military for rebreather training/simulation. They don't ice up, they are light weight in the mouth, they can flow large volumes of gas. For redundancy, it could be built in, twin second stages (dh regs were both one stage such as the Mistral and two stage such as the DA) in the housing for example could easily be accomplished. Since rebreathers use dh like hoses etc, some do it seems and some military units as well and so much of modern scuba is style driven don't be surprised if dh regs come back around! N