Which brings us to, should wikipedia have more domain specific wikis? Why does everyone end up on fandom or some other random wiki site when wikipedia is already ad free, hosted worldwide and ain't going anywhere.
Wikipedia doesn't _have_ to be just a encyclopedic overview of topics, it should have dive ins as deep as you want if there are people willing to write it.
A wiki "is" its maintainers. Separate editors — separate wiki. Wikipedia stops where the interest of Wikipedia's editors in maintaining pages stops; which is usually where the interest of another, distinct group of editors in maintaining those pages starts.
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.
Fandom has become incredibly scummy. I only try to use it on my PC where I have adblockers and a userscript to cut out all the BS they push, and even then it's not enough. They've now turned all searches into cross-wiki searches with absolutely no way to tell if something's on the wiki you're already on until you mouse over it. If I go to The Orville wiki and search for Moclans, it gives me that wiki's page on Moclans, but also pages on Moclans from the "Aliens Wiki", a wiki for some weird ironic cartoon drawn in MS Paint, and another vastly inferior The Orville wiki where the only text on the page is "The Moclans are an alien race in The Orville." All these pages look the same in the search box, and there's no way to turn off cross-wiki search (as far as I could see). I get that they gotta make money somehow, but you should not be able to make your site horrible to use while still dominating the market because you're the easiest product to use.
It was alright until you wanted to leave, suddenly you were banned and "no longer represented the community", and wikia employees had wide latitude in doing whatever they could to frustrate any attempt to migrate off of wikia.
Wikipedia doesn't _have_ to be just a encyclopedic overview of topics, it should have dive ins as deep as you want if there are people willing to write it.