I don’t know if there will be a “next” one. I think these vampires will be with us for the foreseeable future. There will be a separate “enthusiast web” (maybe out of the fediverse but I’m not gonna hold my breath) but it’ll never get big enough to go fully mainstream.
Already the real action is happening on chat apps these days. These are your discords and slacks and clubhouses. In them I see the vitality of the old IRC and forum culture eras.
I'd wager $1 that over the next few years Reddit becomes even more of an Attention-Vampire (or tries to)
It's almost feeling Reddit, Facebook, Twitter -- these "social" echo chamber sites are all getting to the contempt phase of life -- the next disruptor cannot come soon enough.