I once looked after a very frail old lady who had rheumatoid arthritis (a very painful condition) was on lots of medication, came in with a bowel bleed. We treated her aggressively (that means everything that could be done was done, not that we were mean!). All the while she was begging us not to treat her. She wanted to die, she'd had enough pain and distress. The treatment involved colonoscopy, blood transfusion, multiple injections and cannulas. I kept apologising and explaining we could do nothing without a prior DNR. I felt awful, she felt awful, we all felt dreadful.
I don't think frailty scores were around then but she would still have been treated. To ventilate or not is a decision reached with much discussion normally, and ventilating someone who will never ever come off and live, is something that shouldn't be done regardless of resources.