Can Elliptic curve Diffie–Hellman (ECDH) shared secret be used as a static encryption key? 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.
This usage is theoretically sound assuming you do not desire Perfect Forward Secrecy, but will exacerbate attacks on the block cipher by providing potentially very many messages encrypted under the same key, and will exacerbate side channel attacks by repeatedly deriving the static secret (allowing many chances to catch small fragments and re-assemble). 

Consider also that if users A and B have a static secret, then party C who obtains either A or B's key, can recover the shared secrets in both directions, and thus impersonate either party, to either party.

Edit: Another notable attack: party D, who does not have either key but can capture packets, can replay any message that A or B sent, and they can send it in either direction at any time, of course the keys will still be valid so the message will appear authentic. This can be just as effective as a full break in many cases.