I find the irony here that a person can give clear instructions when totally compos mentis, but at the point they lose this their wishes from where they were coherent can be ignored because maybe they changed their mind.

So the thought process of the same person is considered more relevant when damaged than when undamaged?

In what other situation would we say "yes, we know the new evidence is very suspect, and we have really good evidence from earlier, but we're going with the suspect evidence just because it's newer"? Particularly in court - for anything other than mental checks a judge would laugh at this argument.