I think you're missing the point, find the one off cure for cancer you make shedloads of money selling that. Something else will take it's place as a leading cause of death, that rquires research and treatment.
We're not going to run out of conditions that need treating at any time in the near future.
And one-off cures are developed for things that you might think would be bigger money spinners if treated symptomatically.
As I mentioned earlier it's possible to treat a lot of gastric ulcers with a course of relatively cheap antibiotics. Until the role of
Helicobacter pylori in the aetiology of gastric ulcers was recognised lots of people suffered symptoms for years and took antacids, H2 antagonists or proton pump inhibitors to relieve the symptoms. Those agents were very profitable lines providing long-term regular income. Some pharmaceutical companies where sniffy and sceptical when the
Helicobacter pylori link was identified, but they didn't supress it. If they had appropriate antibiotics they started promoting them for use in treating gastric ulcers, in addition to the other indications for these drugs.
BTW: techy the incidence of cancer - the number of people who develop cancer each year is around
10.9 million - that's the number of of potential users of any hypothetical cure, every year. If other cause of death can be reduced, lower respiratory tract infections are major killers, particularly in less devloped countries, the number of people who survive and develop cancer may rise, or at least stay stable if prevention becomes better in developed countries.