Being limited to a phone app and requiring phone numbers is severely limiting as well. Matrix is probably the current best-positioned e2e chat app that should be able to encompass more use cases, but it's difficult to convince users to use anything encrypted when alternatives have more network effects and usability.

Network effects always seem to be particularly egregious gatekeepers for social apps, often keeping users on inferior alternatives for years or decades, as coordinating a mass-switch is too difficult (Signal got particularly lucky with a few recent events or it wouldn't have nearly as many users switching to it right now).