I think signal made the right decisions raising the bar for encryption while maintaining extreme ease of use. Random Joe can click on it on the app store and chat with anyone in his addressbook that runs signal within a minute or so, with no expertise whatsoever.

However I see no reason why a similar p2p app couldn't manage similar without a central server. Trick is cell phones (at least on WAN) do not accept incoming connection. Additionally apple/android push aren't good for a p2p transport.

However adding supernodes (like the original skype) that could run on raspberry pi's, opensource routers, and similar embedded devices might just bright the gap. After all the cpu, bandwidth, and memory needs for instant messaging are pretty modest, even for many people sharing a raspberry pi.