“But I question that “ending up with” cancer can be tied directly to your conclusion that most supplements are worthless.”
You seem to have done your homework, which is pretty important if you plan on treating yourself. I suspect you are in the minority of people who take supplements. I don’t disagree with much of what you say which is based on your own research and discussion with your doctor.
While I agree we have a capitalist system where both drug companies and supplement manufacturers promote their product in the best way they know how I personally come closer to listening to my medical doctor, with years of training and certification (and reputation) than a supplement promotor who is not regulated. So far, that seems to have worked out pretty well.
In my mind, the key is what you described. That is look for deficiencies through your blood tests. If you have them and they can be treated with supplements then by all means do it. I mentioned I do take them still, but they are from deficiencies and actually recommended by my doctor.
My previous method of using them was to hear about a particular supplement that would improve my health and buy it and start gulping them down. I think that is what most do — not the research as you have done.
By the way, my cancer was a rare form of blood cancer with no hereditary relationship and actually no known cause — except a genetic mutation that occurs as cells are copied about a million times a day and one goes wrong.
Diet and exercise go a long way to minimizing your chance of disease, but they don’t prevent it. They also make a huge difference when it comes to the survival of both a disease and the treatment.
Good luck.