I think the case is made most strongly when you have a diagnosis for a serious and incurable condition. You can respond to a lot of the concerns people have about advance requests: You don't know what condition you're even going to have, so how can you possibly project yourself out into the future and know that it constitutes intolerable suffering? You do have that piece of it—that you can predict better—and that helps people feel more comfortable, and it actually undermines some of the arguments against advance requests.
I would add one thing in relation to something you've been saying—