No ability to prevent cancer? Ever heard of the HPV vaccine? I had polyps removed from my colon 10 years ago. There's a decent chance those would have developed into colon cancer by now. Prevention is a major part of reducing deaths from cancer. See also the effects of increasing taxes on tobacco products or restrictions on the use of asbestos.
Those are the exceptions, not the rule. But also, you're wrong: most polyps would not become cancerous -- only 5%-10% ever do. As a preventative intervention on otherwise healthy people, colonoscopy is one of the most effective things we do, and it's not that effective [1].
You've also included vaccines (which I explicitly mentioned) and "taxes" (which are not medical interventions), so you've completely missed the point.
[1] The NordICC trial found an impact of 0.03% on risk of death from colorectal cancer, which was insignificant. It did find an 18% risk reduction for getting colorectal cancer, but this didn't translate into a survival benefit.
