There's an older version of this argument in Schneier's Applied Cryptography (1996). He also concludes that a 256-bit key is secure "until computers are made fr

There's an older version of this argument in Schneier's Applied Cryptography (1996). He also concludes that a 256-bit key is secure "until computers are made from something other than matter and consume something other than energy", IIRC. However, despite things like Ed25519 using 512-bit curve points for 256-bit security (you lose a factor 2 off your exponent because math), this particular instantiation fails much harder if a quantum computer running Shor's algorithm ever becomes reality. Meanwhile, 123456 still tops the password charts wherever it is allowed.