I don’t see something else naturally taking its place either.
I like the idea of Discord as a backchannel but it suffers from the problem that it's a relatively niche app, and no-one is going to install and learn how to use it just for a conference.
I think that Discord would work well for a learning community though.
Sara Winge is the person behind Foo Camp, O'Reilly Media's pioneering "unconference," which we launched in 2003, and which has spawned hundreds of related events known and loved by passionate geeks worldwide, from Bar Camp to City Camp to ORD Camp.
Some historical perspective.
The pocket guide to organizing your own Barcamp
Sounds to me like there's room for new models for large events. One approach could be to adapt the BarCamp-style unconference to online venues.
How do we prevent putting annotators in a bubble while still enabling annotators to discover annotations they care about?
unconferences
A good resource on the "unconference" is Michelle Boulle, Mob Rule Learning.