I totally feel it when you say you lost him years before his death date. It’s terrible. My loss still feels so fresh. Thanks for mentioning your grandfather; it is a comfort to know someone else knows what it’s like.
It was basically the end of flowers for Algernon on a longer time scale.
I don't know what the US law is, but in the UK at least even flying to Dignitas (for example) with someone is legally not kosher. I didn't pry but I think this situation lead to a suicide in my family also (attributing a single cause to a suicide is usually not a good idea so I won't ponder, other than that it wasn't clean or dignified.)
I think the same argument can be made for terminal illnesses also but I think the issue there is extreme pain rather than loss of mental faculties which is potentially not the same thing in the eyes of a court.
It's terrifying watch him lose his memories week by week.