Let's consider these alternatives as hypotheses
Some people are dubious that the pharmaceutical industry would be motivated to find and produce inexpensive treatments for cancer (as alternatives to current treatments which are lucrative), preferring that they wither on the vine for lack of development.
Does that suspicion make it true? Nope.
Others appear to be dubious that for-profit corporations could ever deliberately fail to develop effective treatments.
Is it credible that an industry would fail to support potential treatments which had the potential of reducing their profits substantially? Probably; that's not outside the range of observed behaviors and does not stretch credulity. It doesn't need to be as dramatic as having taken a working treatment through all clinical trials and knowing it's safe and effective then suppressing it; it wouldn't get that far. More likely if this happened, it would be more subtle, like discounting and dropping any promising leads early if they did not show promise of increased profits. In which case we may not know if those would have panned out or not.
It's not tinfoil hat time to suggest that an industry might be motivated by self interest (or even to suggest that it's their proper role to do so, safeguarding their shareholder's interests). All that means is not to dismiss the possibility out of hand, but neither should we believe it without further investigation.
Notice that this can co-exist with the same companies also doing positive things in the world (and making a good profit thereby). They need not be all evil (ignore any positives) or all wonderful (ignore any negatives), and we can approve of some parts of the system as it is and disapprove of others.
As an aside, we are talking about alleged "effective treatments" for some cancers. It's good to be careful about "cure"; and no one treatment is likely to be effective for all forms of cancer.
It's not very credible that there are working alternative "cures for all cancers"; it is credible that there could be inexpensive and effective treatments for some cancers which are not receiving the support needed to become widely known and approved treatments, because of their potential for eroding profits.
