Right, I suspect that, even if we ignore just how singularly unifying cancer has become as an enemy to every echelon of society, there’s simply no way any individual or group of conspirators could or would keep a cure under wraps for very long.

Like you said, profit is one good reason. As profitable as the cancer industry may be overall, any capitalist agent within the space with the sudden power to “sweep the board” and make an unforetold fortune in the process has little incentive to cooperate with their competitors.

Another reason is the irrepressible selfishness and entitlement of the wealthy. There’s no way wealthy decision makers would choose their own death if a potential cure was near at hand. Assuming the cure was developed in secret, exclusively for the wealthy (which might actually happen at some point) it wouldn’t be possible to maintain such a secret for very long.

Finally, if there is any truth to the speculation that a true general cure would, in effect, amount to a cure to aging itself, then in addition to the reasons above, any supposed conspiracy would face the impossible task of containing perhaps THE most catchy idea of human history. If that was actually possible, we might as well also believe in ancient myths like the Tree of Life, the Fountain of Youth, and the Holy Grail.