I have two parties:
Alice who has a static public key PA
Bob who has a static public key PB
A static shared secret (S) is computed with ECDH.
Encryption and MAC keys are computed with a key derivation function: (KENC, KMAC) = KDF(S)
The two parties send messages to each other, encrypted and authenticated respectively using KENC and KMAC. The encrypted messages are also stored in this encrypted form.
My question: how secure is using these static derived keys (KENC and KMAC) in the long run if I want encryption and authenticity, but I do not care about forward secrecy.