The idea with Keybase (can't speak for signal) is that your client doesn't need to trust the server. You only need to trust that your client isn't compromised, which is easier to verify.
Sure (and that's true for signal as well), that's not my point. My point is you have to rely on that service being available. And you need to trust it to deliver and (at least with keybase) persist your encrypted data. And if that service becomes as prevalent as email, that gives the organization that controls it a lot of power.
Another concern I have with the proposed replacments is many (Signal, Keybase, etc.) depend on a single centralized service.