If cancer was insta-kill instead of the slow-death-money-milking disease that it is
This ignores a basic fact about cancer treatment: standard chemotherapy, radiation therapy, and surgery aren't very profitable for pharmaceutical companies, and for many cancers, that's all we have. They may be profitable for other sectors of the medical system, but these are also a huge drain on the economies of rich-world countries, who have a big incentive to keep costs down. If you get one of the cancers for which there isn't a $100,000/year drug, your only option is a quick course of debilitating treatment aimed at eliminating metastases, which will either work and leave you cancer free (if you're "lucky" and have one of the less aggressive types of cancer, and/or catch it early), or not work, and you'll die in a relatively short time. Or, if you're especially unlucky, the therapy itself will kill you. No pharma company is getting rich off these patients.
If you do get to take the $100,000/year drug, there's a good chance you'll only add a few years to your lifespan anyway. Which is part of the reason why these drugs are so expensive, of course. On the other hand, a drug that could either a) eliminate cancer outright, or b) suppress cancer permanently for as long as it's taken, would be worth an incredible amount of money, either up-front or over the course of decades. And insurance companies and governments would be much happier shelling out hundreds of thousands of dollars for a treatment that might actually "cure" the patient in some meaningful sense (and enable him or her to keep paying taxes and/or insurance premiums!), rather than a treatment that probably isn't going to work over the long term.