I think people who sign DNRs assume that they will come into effect when they are brain dead, rather than withholding life-saving medicine during routine, but serious, treatment.
That’s not necessarily what a DNR does. It is not just a don’t resuscitate order. You can be very specific. Mine allows only for palliative care. It prohibits a feeding tube and specific other measures. They can take out my appendix if needed. Other than that nothing