> This time around, people's smartphones are already the unifier. All messaging apps have notifications, so your phone will tell you what everyone is saying across networks, and you just tap to go straight into whatever app the message was in.

I think you can go even further and say that having different apps for different types of communications is actually a convenience in the context of something like a smartphone. Different apps implicitly cater to a different circle of contacts with distinct levels of privacy, discretion, formality and posterity. If you had to explicitly manage these types of preferences in a single universal app it would be both tedious and error prone. Having separate apps that work is specific ways is just so much simpler to manage, it is convention over configuration.