My dad is a good example of this. He was very adamant he wanted to die if he became debilitated by dementia. Unfortunately he ended up having three strokes in quick succession while being diagnosed for vascular dementia and is now a totally different person (with zero recollection or connection to his previous personality or memories, as far as anyone medical can tell) living a pretty miserable life in a facility with 24/7 care, zero language abilities, etc. There was no singular moment he could have made the call. He was entirely cogent one day and absolutely not the next.

I would be very happy to sign off on his wishes, but there is no legal or medical route to do so, so whatever is left of him gets to suffer until his body gives up. I support them introducing some mechanism for this one day and would likely use it myself (all my ancestors went through dementia).