Agreed on there being access to other encrypted messaging apps. But that doesn't account for "app-fatigue" where people are fed up with getting yet another application (think Zoom, Meet, Slack, Signal, Discord, etc) and thus you have friction on getting people to use a new app.
For example: I've tried to get friends and family on Telegram and only succeeded in getting a fraction of them. I also feel that they're using it just to appease me and don't use it with others.
SMS/MMS/RCS/iMessage have default apps on phones and people tend to not seek replacement or simply can't (iMessage) This is what Beeper is trying to fix by introducing an app just for Android users to connect with iMessage users, thus challenging the Apple walled garden.
I haven't jumped on the Beeper wagon because I don't know the consequences of this tug-o-war. Would I lose secure messages if the iMessage rug gets pulled out while my number is registered? It seems I would unless I deregister my number. It just seems too volatile to me but I'm a fan of Pebble and wish the Beeper team+founders the best. I'll be keeping my eye on this.