Well, now we know, though I am not sure if dying by anoxia would be better than the collapse of the hull. The assessments of oxygen supply led me to think that had the submersible been intact, the occupants would more likely have succumbed to the cold; and one effect of approaching hypothermia is the loss of mental faculties making it very hard to make the right choices or react to other people properly, even in critical situations. (The ocean floor temperature is about 4ºC.)
Looking back up the thread, which I'd not previously spotted,
– The remark about the claim that the Titanic was "unsinkable" is so crassly repeated over and over again. No-one at the time who actually knew anything about ships ever said that! It was a foolish mis-interpretation by some newspaper hack, of a far more measured and qualified hope in a professional engineering journal. So comments on here about "hubris" are as wrong and out of place as others denigrating people's wealth – and let's not forget most of those who died or survived on the Titanic were not particularly wealthy by 1912 standards, but even if they were, water and cold respect no riches.
– Yes, the ship became a grave; as now has that submersible, but there are no human remains from the original disaster down there. Just pairs of shoes…. (Dr. Ballard's original explorations of the wreck photographed some.) I am less worried about expeditions to view wrecks like the liner, or the Hood and Bismark, than I am about taking anything from them.
– The search area had to be wide both on the surface and on the sea-bed. It was a small object in a vast area of both. Apart from the darkness and natural sediment in the water limiting lamp range, and the difficulties in searching a very wide area of deep, noisy ocean by sonar, there is a slow current across the site so searching for the submersible would have needed to consider it drifting.
– I had wondered what emergency ballast / buoyancy methods this thing had. Jacque Pickard's Trieste bathyscaphe with which he descended to the floor of the Marianas Trench – some three times deeper than the Titanic's site – had pig-iron ballast held, or latched, by an electromagnet so fail-safe, so presumably it was buoyant anyway.
– It may be that we will never know how this vessel was lost, unless the wreckage is raised for analysis and to give the victims proper funerals. I do not know if that is intended, or if it will be left as a grave. That should be for the families to decide.
There are few areas of ocean that are not graves; but at least the locations of the Titanic, the Lusitania, the Hood and the Bismark, are known. Many more are unknown.