Thanks for the proposal. It looks like Discourse can provide a good place for this. Key is to get people to move over. But before moving I would like to go through this in detail during a PSC meeting. And I also welcome feedback from existing subscribers to the User mailing lists.
The other issue I still have with Discourse is that there is no clear classification on the home page yet. It would be good to have a general “OSGeo Projects” category at the highest level for OSGeo Projects. This can help to avoid the lists getting lost for new users that log on to Discourse.