Most people don't write wills. Their assets shouldn't be lost forever as a result. That would be terrible.
It would be better to opt-in to auto-destruct-when-i-die, not opt-out. It's more of a special case. E.g. create encrypted notes for super secret stuff you want to die with you, but let the default security for photos and documents be "private but recoverable in the event of death or forgotten key."
Not to mention, writing that password down in a will would be pretty bad from a security standpoint while you're alive.
Most people don't stick their wills to their monitors with post-its (there are other secrets in there after all, and many people would like to know those); the legal system has mature tools that are surprisingly good at keeping such secrets secret until the release conditions are met. A will is a Solved Problem, with highly reliable solutions - consider the ways to prove that it is indeed to be opened. Contrast with most computerized solutions and "solutions" thereof, mostly hinging on some form of dead man's switch.
It would be better to opt-in to auto-destruct-when-i-die, not opt-out. It's more of a special case. E.g. create encrypted notes for super secret stuff you want to die with you, but let the default security for photos and documents be "private but recoverable in the event of death or forgotten key."
Not to mention, writing that password down in a will would be pretty bad from a security standpoint while you're alive.