A significant chunk of the rise in life expectancy is driven by a fall in infant mortality, is it not?
"In other words, the individual incidence of cancer deaths has actually fallen."
Cancer rates are strongly driven by age- cancer incidence increases exponentially as people age (stochastically, of course).
I also believe many of the cancers we successfully treat would be non issues if left alone. (all the young women who found lumps and become breast cancer survivors)