[jeroenhd]

They can't implement RCS for every carrier, but they can implement it for some. Others require nothing more than a network call to a predefined HTTP address over a data connection.

Although Google may dislike it, I don't believe Google's RCS implementation (the one most of the world uses) requires elevated permissions. Their activation sequence is proprietary and closed source, though so I'm not 100% sure.

Just having a base implementation for custom ROMs would be a start. Given that Google will block custom ROMs from RCS in their app (I believe because the spec says to), there is a need for an independent implementation if this makes RCS popular enough under American users.

Someone will have to inventory the activation mechanisms for each carrier to see if there really is a disadvantage. I'd start with mine, but all of the carriers in my country have shut down their RCS servers a decade ago.

[ExoticPearTree]

I am not familiar with RCS, but considering the plethora of messaging apps available that just work, why did they choose the most complicated way for users to use it? And what do you mean it is not available on every carrier and needs SIM access?