Any replacement for Discord is going to run into the AIM problem. Even years after nobody was on AOL, AIM remained the biggest instant messenger client simply because it had the most users. A big factor in it losing its dominance, though, was Trillian. Once you could have all your accounts in one place, it kind of made it starkly obvious which ones were redundant. At the same time, it made the barrier for entry feel lower.

Instead of needing different clients for ICQ, AIM, Yahoo, and MSN, you could have it all in one place, packaged with a totally garbage IRC client. So if you had friends on, say, ICQ, there was little reason not to register an account.

This is what we need with Discord. A client that people can migrate to because it's objectively better, which allows them to connect both to Discord and to an open source Discord killer (a Disczilla, if you like). That way nobody has to convince whole ass communities at a time. You can slowly osmosize over as the client gets popular without having to have that critical mass from day 1 to draw people.