Actually, I stopped to think it over. I might have to agree, depending on how much the dog would feel like shit vs the likelihood of a full recovery and also depending on the age.

The dog doesn't have a say in the treatment, nor an explanation of what's happening. If the vet and I discussed and determined that the outcome might be cruel for not much hope of gain, I'd make a plan with the vet to let my pal suffer the least amount possible.

This is far from a blanket endorsement to euthanize dogs with cancer, which should go without sayin.