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.
I mean I get that people make their own choices. I just am confused that more people aren’t interested in having a death of their choosing. Or even consider it. Like, I have a song that I would like to hear as I’m dying. I want to think and reflect on things and feel safe in spite of the incoming end. I am convinced that with a better cultural mindset we could make death a happy, celebrated, communal event that people look forward to in a bittersweet way. The way most people die to me sound sad for all involved, including the dying.
If you like to make decissions you can make and implement them now.
The thing is decissions are tiresome. You might want to forgo some avoidable decissions and focus more on pleasures of living.
I find this to not be self consistent. you would trade two boring work days for one awesome day right? Two bad weeks for a happy one? There has to be some long tail of uncomfortable days that you would rather enjoy on a shorter time frame, even if the long tail is unknown length. Might a warm send off not be preferable to difficult times ahead?
Sorry for your situation.