@taavi great post. The only thing I'll add is a use case: our team (and some others) also need a wiki with *content* as similar to production as possible. It looks like wikireplicas might be solving/have solved the backend for this? But if you're building a skin, or a discussion parser, the wikitext parser itself, etc, it is helpful to be able to test in a realistic environment on terms of the entire template-stack that is active on the main wikis (and for multiple languages, and sister projects, etc).
So something like patch demo -- but which could also be pointed as a CoW wiki replica or something similar so that when you bring up the code you're not looking at empty pages.