That other set of editors could all just be Wikipedia editors, but then they'd have to play by Wikipedia's rules. They'd rather play by their own set of rules, and more importantly, have the ability to define their own rules. Autonomy. Sovereignty.

Now, in theory, there could be some "hierarchy of wikis" all maintained within one system, where different namespaces are maintained by different groups of editors (similar to e.g. Reddit with subreddit moderators) — but, because the goal would remain the creation of a single cohesively-presented encyclopedia, this would result in terrible inter-group conflicts about things that don't fall crisply into the magisterial domain of one group of editors or the other — e.g. rules for when a wiki page in one namespace, should link a topic of a wiki page in another namespace, and how that citation should be done.

(Imagine if editors in namespace A believed that a page in namespace B really should exist, and so kept linking to it, despite the editors of namespace B disagreeing; and the system hosting all of these constantly bubbling up the non-existent page to the attention of the editors in namespace B because it received new external links.)

The solution to this is decentralization. No hierarchy, no shared system, just reusable open-source software and federation through hypertext linkage entirely controlled by the origin. Which is exactly what you get when each wiki is its own website.

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