Multiple back ups and redundancy. For my main cold storage, I have my seed phrase backed up on paper in two separate places and my passphrase backed up on paper in two separate places. Add those back ups to the actual wallet itself (which is encrypted and on a permanently offline device), then I would at a minimum need to suffer complete and simultaneous data loss in at least three geographically separate and very safe/secure locations to mean I lose access. This is incredibly more robust than having everything stored in only one place, especially a place so fragile as your brain.

You are wrong here. There are hundreds of reasons someone might suffer memory loss, and many of them are completely unpredictable and can happen to anybody at any time with no warning. Everything from head trauma from a simple trip or fall, through to an aneurysm in your brain you didn't know existed rupturing. Even with slow onset memory loss, many people don't realize their memory is fading until they've already forgotten significant amounts of details, by which time it would probably be too late for you to access your coins. 15 million people have a stroke each year. 70 million people suffer a traumatic brain injury each year. 10 million people develop dementia each year. That's an awful lot of people with the potential for memory loss. I don't like those odds.