The use of AI, if it does ever happen, will be used in both directions.
The systems will not stay at AES256, newer algorithms will come out.
AI may be used by some to try and hack it, but there will also be AI behind encryption keeping things more secure.
Encryption does get periodically updated. Used to be frequency inversion scrambling was "good enough", then DES, now AES. It'll keep rollng along, trying to stay one step ahead. Agencies should also be rolling their keys periodically, so anyone discovering the key shouldn't have much time to enjoy it.
As for AI and those developing it.
I'd like to think they are "responsible enough", but we know that is not the case. Someone is going to see a way to make money, and there won't be any amount of rules/laws that will stop them once they smell the green. It -will- get used and abused in the name of making people richer, and that will come as a detriment to our society.