Would be nice if forums come back simply because of the searchability. The problem is when they become inundated with users and have too much noise. Maybe it will be just another cycle which will lead back to Slack/Discord after some years and then back to forum after some more.
When a forum is used to help someone fix a problem, there is a communication thread that (as you say) can be searched and followed by other people. In things like Slack/Discord/IRC, this kinda of thing "disappears". Even if you have seachable logs, I think these are a lot harder to search than a forum that is more "structured".
I do believe there is value in having both forums and Slack/Discord/IRC.