>there's no "be a better/stronger cancer and spread more effectively to more hosts"
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.
Right, but that's within a single host isn't it? Like patient A gets that mutation and succumbs, that sucks, but the stronger cancer cells don't them jump to patient B the way antibiotic-resistant bacteria do.
(barring the transmissible cancers article that your sibling comment linked to, but that's not the common case)