That's exactly what most people say.....as long as they've never walked in others' shoes.
And they've never had any health issues that involve constant severe pain & a hopeless outcome.
There is nothing easier than saying "I would never........" when they've never been there.
You raise a related but distinct issue--end of life choices. While some very religious people might see having a DNR, refusing treatment (surgery and/or Chemo) for a cancer as a kind of 'suicide', i see that as a separate thing with different driving forces. i don't know for certain but i suspect people more often make such choices after an at least average length life-span, when the chronic age related body changes are already taking a toll on one's comfort and endurance, sapping energy just dealing with chronic aches and pains.
Recently a very dear friend made such a choice. We'd became friends in Junior year of High School, drifted apart around age 26, reconnected on FB in our early 60's, she was just 2 months younger than me, and except for catching up on the 'meantime' it was almost like no time had passed we talked easily about everything and with our same old empathy and understanding of each other. She had been having strokes for some 4 years, and the last one this spring affected her ability to articulate what she was thinking accurately (tho close relatives and friends could still decipher it). Then in late June she was diagnosed with colon cancer. She moved into a hospice (fortunately her parents left her quite well off and she had used/invested wisely so $$ not an issue) refused any treatment for cancer, only palliative care for symptoms of her health.
The long phone discussion i had with her about all this in late June, was our last. She explained her reasoning, talked some about the 'amazing' life she felt she'd had, how happy she'd been to be able to help people but that she felt it was 'time'. The doctors and a few people tried to argue with her about it, but those of us who knew and loved her, understood. i've had that conversation, as everyone should, especially as we get older, with my kids--they know at what point i would consider life untenable any longer, and would want to be merely kept physically alive. i told her understanding, accepting her choice didn't mean we'd miss her physical presence in the world any less. Monday i got a call from her Executor, she had quit eating in late July, (as my Dad had two weeks before his death at home under our care) and she died on 8/13.
Some might think and/or say that the course she took was suicidal--because doctors might have prolonged her life--but at what cost to her physically and emotionally? i firmly believe that when it comes to that point for any of us we have a right to choose to let nature take its course. i would also ask people who view such choices as covertly 'suicidal' if they pass the same judgement on people who continue to smoke or drink despite doing continuing severe damage to their bodies by doing so. For me they too get to make that choice, tho i've been known to tell those who made the latter choice that i would not sympathize with them over any symptoms, i didn't want to hear complaints from the effects of those substances on their bodies--because they knew the potential consequences of continuing because they were already experiencing some of them .