@aklapper
Part of the argumentation is based on a wrong assumption as Wikimedia Phabricator is not "used in numbers". Please compare the number of Wikimedia wiki editors and Wikimedia Phabricator users (minus some developers only). There are still many on-wiki folks who are uncomfortable using Wikimedia Phabricator.
But what is the alternative, on-wiki discussions? Does this actually work or can it? E.g. I have a question about a class of OOUI. Where do I go?
And how probable it is that right people will come across that topic? Especially if the question is esoteric and only few people know how the component works. The devs who write them are often more known by their Gerrit/Phabricator handles, not wiki accounts.