I think this is great. I've been part of a few of the mentioned communities that transitioned to Discourse, and a few others as well. The most positive change has been the increase in community participation.
As someone initially more comfortable with Mailman mailing lists, I empathize with people who have strong email workflows. Moving to Discourse isn't painless. (There is a "mailing list mode", which helps some, but not entirely.) A web UI has some downsides compared to text, for those so inclined.
But there are upsides as well. Threads are more isolated from each other, and are effectively dynamically-created mini-mailing lists. You can edit your posts, removing typos or clarifying points.[1] Worrying about top posting becomes obsolete. (I recognize the irony here, but I am on mobile, so....)
But like I said, the biggest upside is improved community participation. As an anecdote, I know people who cared deeply about a particular project, but never felt comfortable enough with the mailing list interface to chime in. Forums, for whatever combination of reasons, are more approachable. (I even feel this myself, which is perhaps why the anecdote resonates with me.) Plus, Discourse provides richer means of communication through reactions, and it is better equipped to facilitate moderation and disputes.[2]
I'm not sure what prompted me to write this fawning email, except that I wrote a very similar one when Nix started their trial, and now they've switched away from their mailing list completely and everything seems to be going quite well. I encourage people to give Discourse a go!
P.s. Discourse would never replace all means of communication, but it could potentially replace all discussion-based mailing lists.
[1]: The ability to edit posts can also be abused, but in practice this rarely happens.
[2]: Moderation can also be abused, but its absence is more frequently the problem!