The most legally that can happen at the moment is a Do not resuscitate order, which is usually taken by a doctor (or the patient prior to their illness) or somebody with power of attorney. And yep I agree, what's the point if somebody's quality of life is so poor that they have basically none.
As for paying for care. This is an issue that my family are going through at the moment with my Dad. He's going to end up in residential care at some point this year, he will have to pay a contribution towards that as he's been assessed as having a high income and assets. My parents aren't rich, they bought their council house in the 80's, sold it for a mint and then bought a nice house in sussex before they moved here. My Dad has a good company pension too, so they are well over the threshold.
I've got legal power of attorney over my dad (and mum) so I've split their ownership of their house into equal 50/50 share. Before that they owned 100% jointly, so they would have taken the full value of the house to pay for my dad's care. Now they can only take up to 50%, but yes it does feel like a kick in the teeth for somebody who has worked his whole life, paid his taxes and been a decent hard working man to suddenly have what he was going to leave to his kids, suddenly taken away from him to pay for his care.