You are right, this is about setting the default (though Google Groups will be going away).
I should be clear, Matrix isn’t going away. My team and I are all active in there there every day, and that’s not going to change. This is more about defaults, as you correctly state. It’s more that the number of mentions/links to Matrix across out GitHub repos & documentation will likely massively reduce,
I think ansible-inclusion actually make very good use of GitHub Discussions (threading, checklists, ability for reviewers to update the top level checklist). This process seems to work well, so I see no reason it should change. If it isn’t broke, don’t fix it
Google Groups will be going away, they aren’t in a good state. It’s a difficult platform to use, and it overlaps directly with The Forum, and offers far fewer features.
If there are repos with GitHub Discussion (ansible-lint for example only has 3 posts this year), then I feel it’s OK to archive those.