I’m pretty sure in the uk it’s illegal to allow unnecessary suffering of an animal so if an owner withheld the necessary euthanasia it would be something they could be prosecuted over.

People are different, and although we have DNRs and living wills many countries don’t have legal euthanasia. Big debate. I can see different valid points of view.

What I would say is having witnessed many with dementia is that in some cases it’s a gentle decline, others not. Also we may change our mind in later years about our wishes but possibly not be able to communicate it. I’d rather see more effort given into good dementia care practices, because that makes a massive difference to the quality of life.