Saying we can never cure cancer because it's not a single disease doesn't quite strike me as accurate.
I'm not an expert so feel free to correct me, but my understanding is that there are plenty of other animals, such as the naked mole rat, that are effectively immune to all forms of cancer because while it might not technically be one disease, all forms of cancer share enough characteristics that there exist a small group of genes that are able to suppress it or develop an immune response that effectively prevents a tumor from growing.
I'm not an expert so feel free to correct me, but my understanding is that there are plenty of other animals, such as the naked mole rat, that are effectively immune to all forms of cancer because while it might not technically be one disease, all forms of cancer share enough characteristics that there exist a small group of genes that are able to suppress it or develop an immune response that effectively prevents a tumor from growing.