These numbers have nothing to do with the technology of the devices; they are the maximums that thermodynamics will allow. And they strongly imply that brute-force attacks against 256-bit keys will be infeasible until computers are built from something other than matter and occupy something other than space.
Long story short, brute forcing AES256 or RSA4096 is physically impossible
That’s true for AES-256. But brute force attacks are not the most efficient way to attack RSA, so it’s not true in that case. (Eg quantum computers would break RSA-4096 but not AES-256).
Yes (although practically speaking it’s very unlikely that Grover will ever break AES-128), but that’s still a brute force attack and still subject to the physical limits mentioned in the Schneier quote. Whereas attacks on RSA like the number field sieve or Shor’s algorithm are much more efficient than brute force. (Which is why you need such big keys for RSA in the first place).
