They moved to the current Signal model because this was leaving metadata unencrypted (even if the content of the messages was encrypted, the two ISPs would know

They moved to the current Signal model because this was leaving metadata unencrypted (even if the content of the messages was encrypted, the two ISPs would known X messaged Z with an encrypted message). Opportunistic encryption is also considered bad UX-- too easy to mess up. (edit: typo)