Cisco, Google, Facebook, Apple and others are participating in MLS. The scale of messenger adoption is already much larger than PGP ever achieved, but they are not yet interoperable (like fax) with strong E2E encryption.
Both protocol availability and support in widely deployed messengers will be necessary. Key/identity management will need to pass regulatory/legal scrutiny, but once we have interoperable, multi-vendor encrypted messaging that is usable by mere mortals and globally available at low cost, various groups can start lobbying for regulations that encourage migration to MLS-enabled communication.
The other issue with digital messaging is endpoint security and message archives, after the message has been transported. A pile of faxes is not easily accessed remotely, unlike a disk with message archives.