Earlier quoted context omitted.
One has to already bend heaven and earth to bring people who don't care about Privacy from WhatsApp, Messenger to Signal. Only reason someone is able to do that now is because WhatsApp, Messenger gets into some legal trouble and the media advertises Signal. Preaching about significance of interoperable protocol and suggesting apps which use them is beyond the capacity of even those media.
The solution is always the same, focus on a stellar UX.
Case in point: There was a chat app called Hike[1] in India run by the son of the leading Telecom Billionaire. It had more features(free SMS, Stickers) and arguably better UX than WhatsApp according to its users(100M). But it could never gain over WhatsApp's initial market size in India(Why change what works?).
Final nail on the coffin for Hike was when WhatsApp was made available on the 4G feature phone released by a competing Telecom operator and loads of people got to experience WhatsApp on their first ever Internet enabled compute device.