Chemotherapy is the known cure for cancer, so a more accurate statement would be saying, "Pain killers are pointless, just find a cure for the cancer."
But no one is saying painkillers is pointless. Parent is saying painkillers should be temporary. I suspect they forgot to mention the "in an ideal world" bit.
No, that doesn't really make sense. Note in your example of cancer that pain killers are in fact given to cancer patients, they're one of the biggest users of our strongest pain killers. But nobody claims painkillers treat the cancer. So how could we claim pain killers treat other illnesses such as fibromyalgia?
Chemotherapy is a treatment that targets the actual cause of the pain (the cancer) and (hopefully) kills it, leading to remission and (hopefully) return to normal function of the person as a cancer free individual.
A pain killer is something that does not target the cause of pain, does not treat or reverse or cause remission for that cause, and does not cause the individual to be illness free.
It just makes the person feel less pain, so they can be more comfortable.
But no one is saying painkillers is pointless. Parent is saying painkillers should be temporary. I suspect they forgot to mention the "in an ideal world" bit.