"Where I'm going with this: is it reasonable to assume that "future technology" 20 years from now could crack AES-256 or PGP?"
There are a few related issues here, and so the answer is a bit complicated.
The only evidence for the security of AES is heuristic, based on testing the output of the cipher to check for properties that secure block ciphers should have. Some new attack strategy could completely undermine AES. Similarly, PGP relies on block ciphers and hash functions that are based on such evidence.
On the other hand, public key cryptography has proofs of security under certain assumptions about the complexity of certain problems. A proof that P != NP is necessary to prove that PKE is secure, but it is not sufficient on its own and we do not even have that much.
Now, assuming that (a) the heuristic evidence for AES and various hash functions is a reliable indicator of security and (b) that the assumptions are computation complexity are correct, then both AES and PGP can be used essentially indefinitely. The reason is that your key size can continue to increase -- for AES, you can iterate the cipher (e.g. "triple AES"), and for PGP you can keep making your keys larger (16384-bit ElGamal?), and you will always be able to stay ahead of your opponent. There are issues with this approach, of course -- it would take a lot of computing power to actually use 16384-bit ElGamal, and eventually it would become impractical, which is why there is so much interest in elliptic curve crypto (which allows shorter keys to be used for the same level of security).
So the answer is, "Yes, from one perspective, No from the other."
There are a few related issues here, and so the answer is a bit complicated.
The only evidence for the security of AES is heuristic, based on testing the output of the cipher to check for properties that secure block ciphers should have. Some new attack strategy could completely undermine AES. Similarly, PGP relies on block ciphers and hash functions that are based on such evidence.
On the other hand, public key cryptography has proofs of security under certain assumptions about the complexity of certain problems. A proof that P != NP is necessary to prove that PKE is secure, but it is not sufficient on its own and we do not even have that much.
Now, assuming that (a) the heuristic evidence for AES and various hash functions is a reliable indicator of security and (b) that the assumptions are computation complexity are correct, then both AES and PGP can be used essentially indefinitely. The reason is that your key size can continue to increase -- for AES, you can iterate the cipher (e.g. "triple AES"), and for PGP you can keep making your keys larger (16384-bit ElGamal?), and you will always be able to stay ahead of your opponent. There are issues with this approach, of course -- it would take a lot of computing power to actually use 16384-bit ElGamal, and eventually it would become impractical, which is why there is so much interest in elliptic curve crypto (which allows shorter keys to be used for the same level of security).
So the answer is, "Yes, from one perspective, No from the other."