Everyone thinks we don't need another one until they see one that does something they like but hadn't realised would be useful. WhatsApp with its free messages, Twitter with its enforced terseness, Snapchat with its take on ephemerality - they were all adopted by millions of users who, before seeing the app, would probably have declared they didn't need a new messaging app. Yet they installed it, tried it, and continued to use it.
People will always want to try new ways to message one another. Consequently we'll always get new messaging apps.