Indeed, there's no "be a better/stronger cancer and spread more effectively to more hosts" the way there is with bacteria or a virus. It's not like the flu where we need a new shot every winter because every winter is a new flu.

Once we solve the cancers we know about, they're solved forever, with the one caveat that more people will live longer, so that will increase the window for eventually still ending up dying to one of the cancers that happens to have a non-evolved built in resistance to this or that treatment. Which is a great deal of course, especially if it's a treatment that sounds way less destructive of QoL than chemo, radiation, etc.