I think this is the difficult step. How does an attorney know whether you are incapacitated? Do they video-call you every day, and check that you aren't under duress or being deepfaked? Do they check government records of deaths every year?
It would seem that any system with a reasonable cost and reasonable latency will have to involve software at some stage, so saying "get an attorney" really means "don't trust software you've written yourself, get an attorney to write it instead".
(Okay, the attorney would pay someone else to write the software they use, but in any case, suggesting reliance on an attorney leaves unanswered the technically interesting part of the question, and doesn't really allow us to speculate about what role would actually be left to the attorney).
You realize that the system to deal with this already exists? It's called the legal system, and it hooks into various specialized professions, including medical ones. As in, the partner brings the lawyer some papers from doctors that say you're out of commission, and the lawyer executes whatever procedure you prescribed for such a case. The same way as when you're dead, just without the death part (hopefully).
(Cue HNers saying that the papers can be forged, etc.—if that's the worry, one has more immediate problems in their relationship than the arrangement of a ‘dead man switch’.)
Sometimes I have to wonder if a bunch of hackers from ~2000s decided to live entirely in computer systems, and forgot that the ‘hacker ethos’ pertains also to researching and using existing systems.