There is literally no such thing as a peaceful death. It may sound poetic, and it may be comforting, but it's not true or real in any sense to the people actually dying.
The reality is, it's peaceful to the living, and so we falsely optimize for that.
This seems to be in the realm of philosophy, meta-science and religion. I don't really want to bring the discussion there, but I simply need to point out that many, many religions (and atheists) have different opinions on this matter. For example, a dignified death may bring you closer to the creator (emphasis on "dignified", because suicide doesn't count for some religions).
And I don't believe there's any way to properly gather evidence about what happens to someone after they die. And without establishing our religious ideals as ground rules, I don't think we can really have a good discussion on the matter.
Pretty much every healthcare worker I know, religious or non-religious, leans towards DNR over "Full Code" on themselves. To the people who have watched others die on the operating table, they do not believe that the final moments in a hospital is worth it.
Death is inevitable. You can either have it with dignity, perhaps a week or month earlier than usual. Or, you can drag it out for weeks or months in a hospital (possibly bankrupting your family). Regardless, you'll die eventually.
There are plenty of stories about brain-dead patients staying in a hospital for years (!!) while their bodies atrophy and inevitable brain damage prevents any hope for a reasonable life afterwards. Modern science can keep someone technically alive, but at an unfortunate cost.
Its far luckier to die peacefully (and suddenly) in your sleep... without dragging out your life. Doctors for the most part know this, but they don't want to change American culture or trample on people's right to choose how they die. But its pretty clear what option is preferable.