1. Whoever you're trying to message doesn't have Signal.
2. Signal takes a wrong turn and perhaps starts doing something you don't like as a user and now you gotta find a new app to trust (see "Whatsapp").
> Signal takes a wrong turn and perhaps starts doing something you don't like That's a concern.
Perhaps for you, but definitely not easy for everyone. There's also the case where you don't really know the person you're messaging.
edit: Re Apple, I agree, if they don't implement it then we probably won't see a change. But IMO, the government should play a role here by enforcing a better open standard.