> You think a guy smart enough to give birth to Bitcoin, and maintain complete anonymity while doing so and while communicating in public forums etc, would forget his private key?

> You just need to remember one seed and suddenly you have billions of dollars of value stored in your brain. I don't think that's a realistic possibility.

BIP39 mnemonics ("seeds") were not around in the early days. Bitcoin core software generated random private keys that were completely unrelated to one another. So it was necessary to backup the wallet file each time that new keys were created. Most users would generate hundreds (or thousands) of keys at a time, to reduce the frequency of backups.