Let dead things die. There is nothing noble or moral in suffering. But it isn't a simple issue. I don't think we want to make it too easy and we would also have to guard against it being turned into a business for fear of it becoming exploitative, but ultimately, a family and their medical providers should be able to make such a decision in private without intrusion from the state or nosy neighbors.
In truth, I think this decision is quietly made already more often then we would think.
Of course if we had an actual healthcare system and didn't spend an enormous part of our lives polluting our bodies, maybe this wouldn't really be an issue.