This is a very good point. Although the data (at this link) are not isolated, using a GUI window as a ruler, you can see a clear decrease, just not a visually compelling one. And yes, I agree that as you reduce the mortality associated with other diseases, getting cancer becomes more likely.

What would be more interesting to see, in this particular context, is the mortality associated with different disease groups for different age cohorts. For example, how has the treatment for cancer progressed for people between 40 and 50, rather than all cohorts at once.