Mortality is not a defence against cancer. Mortality is a tool that allows species to more speedily adapt to changing environment. Sexual reproduction is another such tool, which ensures enhanced diversity of future generations, so some of the descendants will adapt better and leave more better adapted descendants.
Btw, in this sense cancer is just another tool, that limits lifespan and ensures generations change. Of course, it didn't appear as such, but most species have no natural incentive to develop a resistance to it.
>"Btw, in this sense cancer is just another tool, that limits lifespan and ensures generations change. Of course, it didn't appear as such, but most species have no natural incentive to develop a resistance to it."
Not exactly and i believe there are a rare few that are much much more resistant to it and as such have become subject of research (trough TP53 in elephants or P16 and P27 in naked mole rats)
At the end of the day this natural incentive depends on when the cancer can appear (generally right away at every step of the cell cycles) and how likely it is which probably depends on the turnover and amount of cells of a particular type or in the being overall which one would assume increases as the being grows and additionally how likely it is to inhibit reproduction as it grows.
As it stands i'd say whilst they're not too inhibiting on this front (for example humans are most fertile at relatively young age well before most cancer occurrences become a problem. We've just extended our lifespan quite a bit) they still can be (kids can die of cancer too) and thus an evolutionary incentive against it however minor is present.
Btw, in this sense cancer is just another tool, that limits lifespan and ensures generations change. Of course, it didn't appear as such, but most species have no natural incentive to develop a resistance to it.