That just would allow a malicious attacker to fake being the server, it doesn't actually compromise the TLS session. So you are talking about a much more sophisticated multi stage attack that needs to be actively executed. This wouldn't at all allow them to record traffic and decrypt later.

The certs authenticate that you are talking to the real server, the symmetric session keys that are usually derived from a diffie helman key exchange have nothing to do with certs. That's two separate (but connected) parts of the transaction to build a TLS session.