Network connections are getting faster... Peoples typing/reading speeds are not... One day it will be feasible for someone's phone to receive all messages sent by every other person on earth. They can then try to decode every message till they find one that their private key can decode. You've now made the perfect privacy messaging system - by sending all messages to all users, no quantity of network packet sniffing/…
I wouldn’t call that perfect because if your private key is ever compromised, all of your communication ever is compromised. Ideally data would be destroyed after a while, which isn’t really possible to do if everyone ever has the data. Broadcasting all messages everywhere would also be an incredible waste of resources, even if it was theoretically possible.
When did forward secrecy stop existing?