Well, if we're diving based probability of catastrophic accident, 1:10,000 or whatever, the best mitigation is just don't dive if it's that much of a concern for you. You can be 100% sure of not dying on a dive if you stay out of the water. Sure, I could carry three of everything and have redundant redundancy and a near 100% chance of never having a catastrophic accident. I'm not going to all that hassle and expense for such a remote chance of a catastrophic failure.
I recall an article I read written by an Army Colonel years ago about Infantry soldiers in Iraq. His research indicated that soldiers in Desert Storm would forego equipment and other personal items in order to carry more ammo. I recall I was in that group he described. Uncle Sam issued us all 210 rounds of 5.56. I had that fear of getting in a firefight and running out of ammo. So I had our SAW gunner break me off about 4 feet of 5.56 belt ammo and I stuck it in my ruck. I "acquired" 3 boxes of .45 ammo from the range in Saudi. Even though I didn't have a .45, I figured if I found one on the battle field in a firefight, then I'd have ammo for it. After all, what could be more screwed up than being in a firefight, running out of ammo, finding a .45 on the body of a dead comrade, and not having any bullets for it? I hoarded a few extra grenades so I'd have more than the four we were issued. All that extra ammo and extra weight meant I had to leave out something else out of my ruck. The logic that you would certainly die if you got in a firefight and ran out of ammo made it all seem very reasonable at the time. You wouldn't die if you didn't have a bar of soap or extra pants or a rain coat. But the fact was, we were always resupplied ammo, and I hauled around a thousand extra rounds and a few pounds of grenades for a year that I never needed. I think I dumped it all in the "amnesty box" when we were at the airport in Saudi getting ready to come home. I actually needed the E-tool and the extra socks and the water and the shelter half and the food and the extra uniforms I left off to make room for the ammo more than I needed the ammo I was hoarding. Sure, the POSSIBILITY of ending up in a "Blackhawk Down" situation, cut off, and needing that ammo was always there. It was just that it was so remote as to be unreasonable given the circumstances. As every statistician knows, odds of probability are ALWAYS skewed by CIRCUMSTANCES. The chances of a red 32 being hit on the roulette wheel are always exactly the same on every wheel anywhere in the world....EXCEPT if the wheel is biased (circumstances). If you flip a coin 100 times and it comes up heads 99 times, the odds of it coming up heads the 100th time is still 50/50 UNLESS the coin is shaved (circumstances).
My buddies and I usually engage in the "same ocean same time" buddy system <gasp!>, and the wrecks we dive and spearfish are 50 and 70 feet (Liberty ship and Indra), and so far for the last 17 years it has worked out for me. I just don't have a problem being at 70 feet with no pony and a couple of buddies "somewhere" within 400 yards of me (hopefully). Apparently the "dumb divers die young" adage isn't always true.
I figure the circumstances just don't validate the need.
I guess I'm just comfortable with accepting the risk. Each diver decides what his/her dive style is, and what his/her own idea of "favorable odds" is, and it's his/her prerogative. I'm not putting my "buddy" at any more risk because he's diving the same way I am. I'll usually run into them 2 or 3 times during the 35~40 minute dive and we're all OK with that.
