In principle you are correct, but in practice you rarely can make such choices because you can't possibly know the future.
You can think you are choosing better life by foregoing the treatment, but it turns out that severe symptoms set in sooner than expected and your life's bad anyways. Or you can think you are choosing longer but worse life but the side effects of the treatment don't seem to affect you much, therapy works way better than expected and you get to experience some joy you couldn't wish for, like holding your newborn grandchild.
What people usually do is balance probabilities and go for longer non-horrible life and hope for the best, and when everything becomes horrible there's usually not much left to make decisions about, which are inherently flawed even then.