How would you fix it?

I see the constant rehashing as a byproduct of the fact that the same old topics get reposted over and over again as they resurge in relevance.

I see this as a byproduct of crowd consciousness. Not everyone is going to see a post before it dips out of sight, so it's bound to pop back up again eventually (if it becomes relevant again), and then new people will see the second/third/fourth postings.

As such, the same rebuttals have to be posted over and over again, so that the new audience has contextual information.

It's not an easy problem to solve.

Perhaps it's a variant of Eternal September

https://en.wikipedia.org/wiki/Eternal_September

Threads don't have to start with a blank page. There should be a way to surface background context.

That's not a bad idea, but you would need to have some mechanism to prevent popular (but now irrelevant) information from previous posts from immediately overwhelming the new context.