> one could easily have two-three chat apps and use each with different people
I guess theoretically, but in reality, no. People are going to use one app. Apple realized this with iMessage and it's become one of their biggest advantages in the U.S. market. Texting an iPhone user as an android user is legitimately a bad experience, because apple knows that their users aren't going to download a new chat app so they can go ahead and make the experience dogshit for the out group users.