Matrix.org/Riot.im has all the encryption you could wish for, a modern, useful interface, and a federated model in which everyone can run their own server and talk to everyone else, just like email.
Though occasionally everything slows down and you have to run a query to cleanup (https://github.com/matrix-org/synapse/issues/1760#issuecomme...)
That won't be enough for the average Tor Messenger user. Email's failings were the impetus behind both instant messaging and Tor. Users don't want/need federated models. Security aside, they want a convenient little app that will receive messages instantly while online but doesn't have to remain online 24/7.