The server holds the private key associated with the certificate for a long time. If you don't use ephemeral keys that means that compromising the sever (hacki

The server holds the private key associated with the certificate for a long time. If you don't use ephemeral keys that means that compromising the sever (hacking, subpoena, etc) that will allow an attacker to decrypt any session they recorded which used that long term key. If you use ephemeral keys, the attacker needs to learn the ephemeral private key instead of the long term private key to decrypt the session. The server throws away the ephemeral private key (plus the symmetric session key) after a short time. So a later compromise of the server will not allow an attacker to decrypt old sessions. An attacker who can break the underlying cryptography on the other hand can still break the ephemeral keys used for the connection, since they (unavoidably) learn the public key from the handshake. It might increase cost, because they need to break each connection separately, but at that point the security margin is terribly thin.