That only protects against breaches of the long-term key, not against attacks on the cipher itself, AFAIK. So if in the future they manage to attack AES (or whatever cipher the connection was using), forward secrecy won't help.
If you use 1024 bit DH with a common group (old/misconfigured web and email servers do this) then it is suspected nation states can break the DH, get the shared symmetric key and decrypt all traffic.
For ECDHE over P-256, they would need to wait for a big quantum computer (which will break all recorded traffic that used a non-quantum resistant key exchange, which is all current traffic).