"As far as I'm aware, people have created, remembered, and managed, long and complex strings of letters and numbers for a very long time. To say it is inherently unnatural is simply not defensible. Actors routinely memorize accurately a whole script for a play. Singers sing long songs. Etc., etc."
Ah, but memorizing King Lear is vastly different to a few ten-character randomly generated passwords. Here's two reasons:
1) Assignment. I know roughly what King Lear is about, and so if someone asks me for it, it's easy to go through my brain and start with that one, rather than start with Julius Caesar or HMS Pinafore. However, I have four bank cards with four different PINs, and there's no context to relate one to another, I just have to know which is which.
2) Transition between pieces. Although a ten-character password has less information than an entire script, it isn't organized in a way that our brains appear to be used to. Our brains seem to use something like Markov chains to store information, associating one word from the next. There will be significant events to hang words on, a general order, and other people around who also know the script helping you along (I don't mean prompting, but a given character has at most half the words in a script, in most cases). Compared with that, a12hrnf89bkj%DJ& is not something that humans seem to memorize easily, since starting at a12 gives no real information as to why hrnf comes next, whereas from Julius Caesar, "He reads much, he is a great observer" has information content and so the transition to "and he looks quite through the deeds of men" is much easier.
There's also the fact that King Lear doesn't need to be changed every six months, but anyway.