It's only required to prove an algorithm that solves an NP-complete problem exists, not find it.
Even if that algorithm exists and was found, it could be that such an algorithm is O(n^123456789), which would not break RSA in any practical sense, though it would be mathematically asymptotically faster than O(2^n).
Even if that algorithm exists and was found, it could be that such an algorithm is O(n^123456789), which would not break RSA in any practical sense, though it would be mathematically asymptotically faster than O(2^n).