You're trying to make this into an old vs. new argument, but I'm just looking at good vs. bad. Discourse being self-hostable FOSS certainly ameliorates that part of my criticism, but it still suffers from being a web-only application with close coupling between functionality and presentation, and contrary to your complaint about "retrograde" UIs, I find Discourse's interface to be quite slow and disorganized, and offers a "narrow" view of discussion, in contrast to more mature UIs' "broad" view.

"Message organization" is one of the most significant elements of this particular use case. Maintaining well-organized archives is extremely important, and it's not enough for messages merely to be searchable -- they should be easily browseable by topic and time range.