I think the best chance there is for an alternative to iMessage to take off is to instantly being pushed to all Android phones by Google itself. Joyn is too high up the carrier chain and “texting” apps are too low in the app and market chain.
Google has the opportunity to unify the morass of messaging services: Hangouts, Talk, Messanger, Voice and SMS (I’m probably missing some here too). A company along the lines of MightyText and on the heels of the Meebo acquisition, Google has the ability to do it, they just haven’t yet. Google could make sending messages effortless.
Dear Google. Half of playing, is communicating .