We do the same when the elderly have advanced degenerate conditions. My grandmother has advanced Alzheimer's. She can't recognize her own children, she has no idea where she is or what is going on around her, she can barely walk and often has to be told how to eat. She is constantly getting UTIs, bed sores, other physical ailments. She is clearly miserable and has no positive quality of life. Despite this, we do everything we can to keep her alive and help her. She's basically a full time job for my mother now. There rest of us help, we have nurses visit, etc. The trouble is, I'm not sure what the alternative is. Leaving her to die seems extremely inhumane, and positively killing her seems abhorrent.