An open source messaging app that actually does end-to-end encryption regardless of whatever law? It might be less convenient to use, but people who feel they need to encrypt their stuff, will be able to do it anyway. So the end result will be just less privacy for people who "have nothing to hide".

People use both of them when they want to do something in privacy, instead of sharing it with the whole world.

If you invent a message app that tells everyone it posts every message to a publicly visible website, then I'd agree that people using it aren't expecting privacy. I don't see it catching on though.