I think the biggest existential risk to reddit is their mod structure. There is no effective mechanism for getting rid of bad mods, users just have to spin off into a new subreddit.
Or conservative mods, or greedy self-serving mods trying to profit from their position, or mods that stop moderating entirely until it gets filled with spam, or asshole mods who want to power trip over their crumb of authority.
There are many pathological flavors of bad mods, and reddit's mod system is poor at handling all of them.
