Anyone reading this can make their own Reddit-esque forum on a VPS and serve a few thousand people for a few bucks a month. And if Reddit ever kicks out all the polished app users/old.reddit users, you'll see that start happening a hell of a lot more
I dont know if anything will overtake Reddit for a very long time because of network effects. But discord is probably the best guess. Although I actually think people do want centralization. They want 1 login to 1 website that has everything.
Presumably that's why they haven't booted old.* users yet. They realize that a really substantial amount of their network effects and their moat stem from quality posts by people using computers.
Reddit is about to fuck around and find out, and unfortunately I think they're going to find out that people will just dump everything into even more annoyingly gated-off Discord communities