I thought of BTC keys too when writing this, perhaps I digress, the significance of memorization keeps reaffirming itself to me.
Socrates discussed the forgetfulness that comes with writing, and when you rely on paper or electronics to store information, including keys, you really have to extend the necessities of actual ownership to these materials.
For this reason I rank developing, manufacturing and powering practical, self owned local data storage as one of the main challenges of our time.