Let me address some of your points:

a forum like discourse, which is most definitely not a classic forum.

Linear post structure, sorting based on latest response, so it is a traditional in vein of old BBS/phpBB systems.

Do you really think you could replace the utility of stackoverflow with… Reddit?

Upvote based sorting and nested comment structure means StackExchange/Overflow is closer to reddit than it is Discourse.

It really is not much of a step to take your logic and replace lemmy or Reddit with Discord as a Q&A support and knowledge base platform.

Reddit, Lemmy, and Discourse are all public forums, Discord is a chatroom.

Even simple features that encourage engagement in months or years old threads are massive boon.

Thread necromancy for month/year old dead threads has always been considered offenses to almost every single forum, which is why most forums lock posts after a month or so. It's not a feature, it's a fundamental flaw with the sorting.

with copious features built specifically to enable and support that purpose. Which both lemmy and Reddit lack

I'm genuinely curious, what are some of those features? I can't think of any significant one, outside of tags.

Forums are ultimately shaped by people, so I would say these forum succeeded in spite of the software instead of because of the software.