Assuming you have respect for your pet letting them pass away in pain is probably the worst thing you could do. The last dog I had just stopped eating after 9 years, I could tell she was in pain but she was happy being around me so euthanasia seemed like the only way to end on a high note.
I'm not entirely against holding out to see if their condition will improve but there's a point where it stops being hopefulness and starts being selfishness.