Why the world isn't already using Signal is why Signal is the wrong tool to switch to.

The world will neither like the same messenger nor will it make the switch at the same time.

So you need to give people the choice to choose an app they like without needing to convince their social network to do the same (potentially x-times, because you are not their only contact).

This is why you should switch to a messaging standard such as Matrix, not a centralized messenger.