My grandmother had a fairly catastrophic stroke (she actually died shortly after due to malpractice, this was awful in the sense that it should never happen but was honestly a blessing in the sense that the result of this stroke was either a nihilist alcoholic trapped in a body that doesn't work or something even worse), witnessing that solidified my conviction that people suffering diseases that cause loss of mental function (like Alzheimer's) should have the right to legally commit suicide in a dignified manner (and the facilities to do within a legally and ethically responsible framework).
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 it's possible for a medical+legal panel to resolve issues like that, but the thing that would probably cause me to chicken (should it happen to me down the line) out would be actually doing it sufficiently long after the initial screening (I don't have a statistic but it isn't uncommon for the result to come long before the really nasty symptoms) but before said symptoms set in. In future we can hopefully spot things like this much earlier (I don't know what came of it but I heard a brain scanning hat similar in mapping capability to an MRI - passive only apparently - being discussed in a physics department, apparently past prototype stage)
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.
