You can fool all the people all the time. This analysis supposes that people actually know, and are keeping it a secret. A secret that nobody knows, is safe ...
A secret nobody knows is not a secret. It's just something unknown. Also, conspiracy theories are about large-scale actions. Large-scale action cannot happen without large numbers of people being involved and know something about what's happening. They may not know everything, but they know enough such that any serious digging will reveal the unpleasant truth.
And no, I don't think a lot of cures are being withheld. But capitalist pharmacy ... has no interest in curing people, only in making money. That isn't a conspiracy, that is human nature in groups. Individuals might want to cure something (like my daughter's fibromyalgia) but if they aren't funded, then it isn't happening. And the people with the money, are not humanitarians, the researchers are not different than the scientists at the Tobacco Institute.
The fact that the alternative medicine "industry's" bread and butter is exactly
selling "cures" to ailments that pharmacology can't cure puts lie to this. See, a company would stand to make it big should a genuine cure is available to a big nasty like cancer, because they get every cancer sufferer on the planet after their cure — they'd be the only game in town — and there will always be a steady stream of new cancer sufferers to profit from. That huge influx of cash would absolutely be worth it. For once, the tragedy of the commons (here, the commons is all cancer patients) works to our advantage.
The problem for cancer and other chronic diseases is that the more we study them, the less likely it seems that there is a final "cure" for any of them. In order to say you're cured of cancer, you would have to get each and every single cancerous cell in your body, but the only way to assure this 100% is to kill the patient, because anything effective enough to get all those cancer cells are toxic enough to whack an unhealthy number of the normal cells. Also, any patient far enough along to have cancer has a pool of pre-cancerous cells that, while not cancerous now, are likely to turn cancerous in the future, even if the current cancer is cured. This has all come out in the primary research on cancer, which you need in order to develop all those treatments of cancer that are coming out all the time. Similar results have come out of other chronic diseases, like AIDS — indeed, AIDS has turned from an acute, lethal disease (with life expectancy of a few years) to a chronic, manageable disease within my lifetime due to the billions of dollars thrown at it by pharma and universities, but there doesn't seem to be a way to actually cure
it short of destroying the immune system and dooming the patient to life in a bubble.
You're right that pharma companies don't care about our health, but only about our money, but by the same token, they don't care about other pharma companies either. They will
screw each other over, and actual cures for what was previously been only treatable are killer apps.