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.
No, but there is "be a better/stronger cancer cell and don't succumb to whatever therapy is killing its neighboring cells." It's exactly akin to how dosing isolated populations of bacteria with antibiotics selects for individual cells that are resistant, which then multiply and dominate [0], just like a tumor.
(barring the transmissible cancers article that your sibling comment linked to, but that's not the common case)
The rare exception: https://en.wikipedia.org/wiki/Clonally_transmissible_cancer
Another curious case of "cancer being an organism" is the HeLa line derived from cervical cancer cells taken from a woman called Henrietta Lacks.