At that point, they can simply listen to all traffic and, if there are packets that aren't listenable due to encryption, assume ill intent, then be free to react appropriately.
Assuming high enough signal ratios in detecting encryption, it might actually prove that encoded messages, or steganography proves to be more effective at preventing state-level eavesdropping... they won't prevent a determined state from getting your secret content, but it could be enough that, if the messages looked unencrypted, they would be passed over for the more obviously encrypted messages -- hiding in plain sight, as it were.
Note, this is not an endorsement of any privacy invasions whatsoever, but musings on what a post-4th-amendment-obliteration world might look like for the privacy-minded.
An inter-country agreement makes us interoperable. All comms are monitored for anything looking encrypted but not decryptable by the state using their keys.
It will be an easy sell because those who want full personal privacy will be seen as suspect. It will also make money from licensing.
Only for their own citizens and trade partners that cooperate with their unrealistic demands.
There's no way a single country can block secure, end-to-end encryption forever, but they can sure make things hard for a while.