That plugin hardly turns Discourse into a full fledged Q&A site. It merely adds a button to mark a specific post as "The Answer". While that may be sufficient for many, there is a reason that StackOverflow has tags, tailored search, reputation, closing, etc. That's what they are selling with that $5 price tag.
It's funny you refute that forum add-on, as it is a good add-on, but it's also just an add-on. A band-aid, not a solution. Too often I see the selected answer "pulled up" to the top, but it's actually just the tail-end of a longer conversation, where the context of the conversation is critical to understanding the answer itself. Not to mention that comment responses (to both answers and the question itself) are interwoven together like a big bowl of spaghetti.
Meanwhile, I think StackOverflow, while great, is not living up to it's potential. I'm currently brainstorming/prototyping a StackOverflow alternative that covers the Q&A scenario, but adds support for more advanced features. If a question is opinion based, then you could have answers categorized into sides. Yes or No would be a common example. React vs VueJS vs Ember could be some side options in a question. OOP vs FP vs Procedural could be sides for another question. What is the right way to X can have a different answer depending on your "side", or it could be that the sides themselves are in disagreement. Sometimes it's nice to see and understand why another side prefers X. Let both side's top answers stand side by side at the top. Another idea is improved connecting of similar, but unmergable, questions. Another idea to connect questions by dependencies. A user could select some premises before asking, debates could be derailed less often by linking debunked arguements to their open debate page where there could be a large chain of custody on each element and nested prerequisite showing why it's wrong. Prerequisite could also be a word definition ("theory" is a random shower thought in some contexts, but in other contexts it's an explanation backed by so much evidence it's a strong candidate for established fact that could only be replaced by an expanded explaination, not an alternative one). Prerequisites could also be an axiom. In addition to questions, there could also be "claims". "Answers" to claims would basically be supporting evidence or refuting evidence.
And basically it would consist of claims and evidence linking to other claims — and automatically calculating scores based on votes for claims. The hardest problem would be to prevent sybil attacks, collusion and so on, that could compromise the voting system. Perhaps the score system would need to be based on something entirely different than voting, such as random polling to leverage the wisdom of the crowd (https://www.npr.org/sections/parallels/2014/04/02/297839429/...)
If you are interested, message me greg+hn then the at symbol then qbix.com
Not bashing Discourse at all. In fact I wish I saw it in the wild more often.