The bottom line here is Trace created a no win scenario. He also caused a lot of thinking. This is something that will, probably, never happen, at least on the cave / wreck side. Due to good equipment and planned redundency a catastrophic gas loss, of all or most, of your breathing supply is very unlikely.
The readers here, that are strictly recreational divers, can learn the most from the information that is in this thread. With a single gas supply and a few daring feats, this could likely happen to a recreational diver and their buddy, by wandering too deep or a little too far into a place they are not trained for. If you are going deep, learn to use a stage. Put a bottle and reg at your safety stop. There are many things that can be done, but you have to learn about them. Dying is bad, but a case of DCI or worse, a bad CNS hit may be worse than dying. Bolting to the surface is not an option unless too many things go wrong at the same time, which in real life is highly unlikely. Poor planning, of course, can create this situation where too many things go wrong.
Our survival instincts are very strong and if we really got ourselves into a scenario as described, our loved ones would likely come first when we knew there was no chance to help further. I don't doubt a few would try til the very end and even die trying. Again, in this impossible situation, it was way too far to swim out. Or, for instance, maybe a large piece of debis trapped a diver in a wreck and their gas supply was very low and yours was marginal. Most of us would not share gas till we both died. Most of us would hope we could get help and return before it was too late. No matter what our plans are each of us has a strong instinct that pushes us to survive. This isn't about cutting and running at the first sign of a problem. It never was as Trace was specific about the situation. Anything that could have been done was, and the options were runnning out. Ovbiously the divers planned poorly and were both going to have to pay for those choices. By taking time to "What If" we all can take a look at what we do under the water and if changes need to be made.
A few years ago I was in a total siltout that could have gone really bad. I did have plenty of gas and when my buddy was lost I decided to find him, and lead, or bring him out (If he was deceased) as I had a lot of gas before the siltout. Fortunately he followed his training and I mine, and we both found seperate ways out...........together. One of these days I will post the story.
Let's hope that this discussion will show other divers the importance of gas monitoring, redundancy, further training and the dangers of failing to do these.
Thanks Trace.