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.
Yes, this are good ideas indeed, but the problem of to much redundancy is the abilty to be stolen, the places need to be really really safe and commercial safes are way to weaks. You can also have the need to get access to your funds when far from home or other place you store it.
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.
You mark here some points the memory can be an issue also, there is no 100 percent magic method to have those seeds safe forever. I may part my funds in brain wallet and another one on paper with some redundancy. Imo a really long passphrase have to be test at least once a week, and of course in secure offline device.