Most users don't care because there is no way and they're not technically minded enough to complain about it. They would use it if available. I can cite many examples of non-technical users who had conversations like "do you have whatsapp", "do you have viber", "do you have X". From that it's safe to say there's a need for interoperability, and to be honest, if messengers get more important, some regulatory body will force it like they did with other things in the past.