Yes, this is the big problem and there would have to be a lot of safeguards but as an individual at the moment I have the right to refuse treatment and even the right to end my life should I wish. I would be happy to put these wishes in writing, witnessed by a solicitor or doctor, and hope that these wishes would be carried out if I was no longer physically or mentally capable. A general bumping off elderly relatives is probably not desirable
but there are circumstances where all hope is gone when surely with relatives consent two doctors could make the decision.
My nephew was in a persistent vegetitive state for 7 months, there was never any question of his survival, his parents could have refused to let the hospital feed him through a tube into his stomach but they would have had to watch him starve to death as it was they watched their much loved son waste away and turn from a lovely boy who just looked asleep to looking barely human. Impossible choices. Sorry 