And lots of people drive drunk and have been OK so far. Does that make it a good idea?
In many of the cases you describe, it is not a matter of a black and white matter of "here you are OK; go over that line and you die." It is a matter of determining probabilities and deciding where to draw the line where the risk becomes too great. Here are some examples.
- Oxygen toxicity under water will kill you. It is caused by breathing too high a concentration of O2 for too long a period of time. What is too much? What is too long? It seems to vary by the individual and the incident. Standards for a safe PPO2 have been created because we are sure people are safe at those depths and those times. We are not so sure beyond those limits. It is not like you will die if you exceed your MOD for a few minutes; but you are not guaranteed to be safe, either.
- Exceeding a certain depth on air is unsafe because of two reasons--oxygen toxicity (mentioned above) and narcosis. Narcosis is very uncertain and unpredictable. Many people have gone deep on air and been fine. Others not so much. Chris and Chrissy Rouse dived to 230 feet on air, and the narcosis led them to make very bad decisions that got them killed. Opal Cohen and friends decided to do a bounce dive to 300 feet on air. When they reached 300 feet, she was so narced she kept going down. Her friend and DM went after her and turned her around. They did not have enough gas to do decompression. She died. He will never walk again.
- No decompression limits are where we are very sure that all but a tiny percentage of the population will be safe. Violate those limits by a minute or two and you will still probably be OK, but the odds are starting to go against it. The more you violate the limits, the more likely you are to have a problem. You could ask Chris Rouse, Chrissy Rouse, or Opal Cohen (mentioned above) about that if they were still alive.
So, in all cases, it is a matter of rolling the dice. There is a good chance you will be OK. There is a chance you will not. The more you push the limit, the more likely it will be that you will not be OK. It all comes down to one question--how lucky do you feel today?
And BTW, the Navy stopped testing people's ability to withstand oxygen toxicity in the chamber years ago, when they realized that for reasons that are not well understood, people can endure much greater PPO2s in a chamber than they can under water.