I think I have said this before on here, but an elderly neighbour of ours (no dementia) was resusciated after a 2nd or 3rd heart attack. He told me afterwards that the after-effects were so painful (cracked or broken ribs etc) that he wished they'd just let him go. This alone was enough for me to want a DNR for my mother.
Quite apart from anything else, I could not think it at all kind to strive to keep alive anyone like she was, already very elderly and with advanced dementia, if Nature was trying to let her go.
Our neighbour did in fact die not many months later.