>There are complex mammals that effectively don't get cancer

You got a source for that ?

That's not true at all according everything I know, but maybe what I know is outdated.

AFAIK there are only mammals that seem to develop cancer much less than they should - namely large mamals like whales. Other than that every animal above and including Cnidaria deveop tumors. E.g. even the famously immortal Hydras develop tumors over time.

That's what makes cancer so tricky. There is good chance that far, far back in evolution there was a selection between longevity and rate of change or something else. Therefore may be nothing we can do to prevent cancer and can only hope for suppression / cures when / if it happens.

Again, this may be outdated.