There are several reasons why Apple keeps iMessage closed down (to the point it resisted adding RCS to its smartphones until the pressure was too high), the main one being that it serves to hook people into the iPhone ecosystem, at least in the United States. Android users have, for years, attempted to circumvent Apple’s unwillingness to make iMessage multi-platform, but perhaps the most outstanding effort was Beeper Mini. If you wanted to give it a spin, though, it’s too bad, because the app might be good as dead.
