We just had our great Dane cross the rainbow bridge a week ago today, which was a super tough decision. He was still having good days, but the bad days were starting to outnumber them, so we made the decision before his entire existence was suffering.
At some point you have to take a hard look at things and decide whether you're keeping them alive for them, or for you.
I'm not a vet, but an elderly dog with dementia isn't going to understand why it's unable to breath, so it might be good to talk with a vet who will give you a straight answer about what they'd do for their pet in the situation.
1, enjoys facials 2, enjoys swallowing 3, hates all of it.