Interesting topic. Few questions/observations.
1.) In some endeavors, like running (sprint or distance) you can strive against a goal, whether externally competitive or a personal best, and pour your heart into it with low risk of failure killing you. You could pull a muscle, be worn out and sore for days, maybe get heat exhaustion, etc…, but unless you have underlying health problems (e.g.: older, coronary artery disease) or go to ridiculous extremes (e.g.: intense long distance run in 95 degree humid weather), odds are you get 'tactile feedback' and harm without serious permanent injury.
2.) In some endeavors, like weight lifting, if you have people spotting you, you can try, and if need be others can step in and safely rescue you in case of a failure.
3.) In some endeavors, like the above, as you progress gradually, the strain/harm progresses gradually, so you can judge fairly precisely what you can and cannot do, and slowly advance as your conditioning/ability improves. Lift a little more, run a little farther...
Seems to me that while pushing the limits is psychologically stressful, you don't get quite the same tactile feedback that some land sports involve. A person may feel okay, though task loaded, until an added stressor (e.g.: free flow) tips the balance like a house of cards coming down, and the diver is overwhelmed and panics. By the time you are 'seriously strained,' the cascade of events may be headed for a fatality. With extreme diving, rescuers may be absent or not spotting you closely enough. Narcosis can creep in, perhaps without diver awareness. Add to that that susceptibility to narcosis (and they say ox tox) varies.
A problem with pushing your limits is it's hard to know just where they are on a given dive & day, and if you push to the point you find them, you're liable to kill yourself once things go that far. How many divers who 'push it' die on dives they've made the equivalent of before?
Let me put it in practical terms. I could ask some athletes how much they could bench press, or their time on a 1 mile run or to swim a given number of laps, and get a pretty set answer. But while a diver can tell me his deepest dive, or his longest, or how far he made it back into some cave maybe, or the coldest he dove…how many divers can tell me how deep, long, dark or extreme they can dive?
Scuba seems like a mighty dangerous place to go hunting your limits.
Richard.