I think it does boil down to a combination of mental acuity and conscious experience. Basically are the lights on in there or not. If yes, human compassion dictates that we should do what we can to help someone in a bad situation and at least try to give them a decent life of ideas and imagination despite being genetically fucked out of a working physical form. But if no, then human compassion almost demands the opposite; a broken or mutated brain beyond a certain point is like a perversion of the human soul, whether you're speaking literally or metaphorically. And if you do want to get metaphysical about it and assume humans do have some sort of persistent reincarnating soul that is "hosted" by the neurological hardware of our physical bodies, then I can think of no more sickening tribulation than being locked in a heavily neurocognitively re+arded body for over half a century. Imagine having your size 10 right foot squeezed into a size 7 left shoe for 50 years and being forced to walk 6 miles every day. Imagine how warped your foot is when it comes out of the shoe. Now of course my "euthanize that mothafucka" cutoff point would be considerably high and my bar for conscious experience is pretty low, as there are plenty of re+arded people who are very clearly still people with feelings and interests and memories and loving families, but as others have said there are some cases where it's literally just like you're infected with late-stage rabies, and we shouldn't try to prolong and maintain that state.