It didn't kill forums. They survived. But it's a good idea to archive them. There are a couple of forums that have been down for 20 years, and I read the archives. The discussion quality is better than anything on reddit. 

Search is not that big of a barrier of entry. Biggest one is prerequisite knowledge. Even if you find good info, you can't make use of it without related experience. E.g. a forum for film photography will confuse most people without knowing the techniques. So most people will stay away from the forum. The notion that half the world will be brought to that level of awareness in any advanced subject is completely ludicrous. Discord doesn't solve anything (unless you count talking to lolis as a problem) 

 Yes I think you're right on most of this.

Some forums have survived for sure and they will hopefully outlast reddit. Some did relocate to Reddit tho. 

 But I wasn't saying half the world is brought to any awareness, they're just potentially unwanted footfall in subreddits  or something  

 That's fixed by skilled moderation. Good luck finding 20 people with enough experience in an advanced subject. If you don't have funds to motivate them to share their experience, it's likely a fool's errand. 

And why should anyone like that share if it's not appreciated by the plebs?