New messaging apps will not change the world. Finding new ways to sell wares to people is not solving big problems. Why do so many entrepreneurs act like they are working on the cure to cancer?
I could see one changing the world. Maybe none of the current contenders, but email is basically a messaging app and it changed the world.
Something equally federated but tailored to 21st century concern (photos, video, encryption, read receipts, etc.) could change the way we talk to people. We have certain forms of in-person discussion (mostly revolving around discretion) that are not well supported (and therefore scale poorly) in electronic media.