A perk but also a logistical challenge. In the small number of attempts I've observed so far, a big problem is that if you start with a small community but all of Wikipedia as your starting content, you immediately get overrun by spammers, because your small community can't police editing of 2m+ articles, and you also lack all the anti-vandalism "immune system" bot infrastructure that Wikipedians currently run.

One way to sidestep it would be if your experiment is with a community structure that's much more restrictive than Wikipedia's. If, for example, only approved editors can edit articles, then you avoid most of the spam problem. But it's not clear to me that insufficient barriers to editing are the main problem with Wikipedia.

It seems to me that Wikia should be an effective disruptor, but it isn't and I'm not sure why. Culture? Stigma? Awareness?

Reasons I think it is: it's very domain-specific, which makes it possible for qualified editors to flock to appropriate domains; there is a separate command structure per Wikia, enabling access on a per-qualification basis; it practically begs you to copy content from Wikipedia as a base; it already has a financial support system in place with the ads, and presumably has solid infrastructure backing it.

I can see how some of those strengths would double as weaknesses, though.