What do you mean by skilled with MediaWiki? There is a visual editor available - I don't see what skills are needed?
Regarding using MediaWiki, it is to my opinion the far best. I see no advantages using Markdown and git - at least as many skills for this is needed as to edit MediaWiki if not more. I would be very sad, if the wiki moved to readthedocs or GitHub Markdown. Could you elaborate on how it is a "hard-to-manage dinosaur"?
Isn't this a concern of setup? That is because it is chosen that an admin has to act - and self sign up on the wiki isn't allowed. It is not just about a wiki and GitHub account; one using GitHub and Markdown also needs to learn how git works (not the commands, but what source control is, branches, remotes, pull requests, commits), install a program (Sourcetree, Github Desktop) and get stuck in different merge issues.
Isn't this the exact issue that makes it more difficult to use than MediaWiki. We can't both say that skills are needed to edit a MediaWiki and then turn to another platform needing twice as many skills as to using a visual editor?