You're absolutely right about this, and it does concern me. I hope that my hypothetical OOA wouldn't be an empty lungs and mouthful of water sort of thing. A free flowing regulator gives lots of cushion. So does a busted hose. I have a hard time imaging my nightmare scenario where I can't go to my necklace reg. Maybe a catastrophic failure in the 1st stage? Maybe simultaneous entanglement and OOA?
Honestly though, this one does worry me. I still think that the chances of the nightmare scenario are slim, and other modes of failure are more forgiving. My primary 2nd stage is on a 7' hose which itself contains a lot of air to expand on the way up. In truth I've only done shallow CESAs during training. I'm almost looking forward to a real world excuse to do one, almost like the first time I lost a mask unexpectedly and was pleased at what a non-issue it was. On the other hand, a real life CESA and a lost mask are very different things.
Thanks for the correction
@rek_diver. I usually finish solo dives with close to half my air remaining. That gives me more extra air than a 19 cf pony. At 500psi an AL80 contains only 13 cf remaining. At 1400psi I still have over 36 cf - a difference of over 23 cf. Again, I don't want to make this into a solo diving debate, but it isn't like I'm not thinking about these things.