I was on several mailing lists that I would occasionally answer questions on when something popped up that I could help with. Most of these moved over to forums over time. I have no interest in popping in to a bunch of different forums everyday to answer questions. I still pop in when I have a question myself but I find the chance of getting a helpful answer has reduced.
Oh no, not Discourse :( I never liked Discourse. I guess mainly for the UI and UX/navigation.
Still likely an improvement for approachability. For a mailing list you essentially need pre-knowledge and a client to use reasonably well. A [Discourse] forum may need an account, but is hosted and straight-forward.
Given that it's already established I see why the question of alternative forms and platforms wouldn't even come up.