If we prove P=NP, it's most likely that it will have zero implications for cryptography and instead mean that our mathematical axioms are bad.
Similar thing to Banach-Tarski. Very interesting to the mathematician, but zero implications in the real world. What it really shows us is that nonconstructive mathematics doesn't provide the best models for the physical world.
“My main point, however, is that I don't believe that the equality P=NP will turn out to be helpful even if it is proved, because such a proof will almost surely be nonconstructive.”
— Donald Knuth