I agree, although there's something to be said about those threads attracting people.
/jp/ currently has 22 threads with two replies, 32 threads with one reply and 31 threads with zero replies. That's a total of 85 OPs and 76 replies, so more than half of /jp/'s threads have an average of less than one reply per thread.
You could certainly point out that they're throwaway funposts that don't need to attract replies, and you'd be right, but the point is that if the thread doesn't have an attractive factor I doubt it'll have an important effect on activity.

Now, how do you get people to make fresh threads, Iunno. When I want to discuss something that doesn't currently have a thread, I make a thread about it and it generally goes well. The odd funpost thread also tends to go well. But it's not an everyday thing.

Back in the 4/qa/ days I would routinely get surprised by with threads of mine would become popular whenever I made a bunch at once to do some cleaning. I think kissu activity, if not all imageboard activity, works exponentially and a thread with even one reply is far more likely to receive more than one sitting with zero.
But, there are a lot of /jp/ threads that I don't really have anything to say to at first glance... maybe I should try harder..

Would simply plotting thread creation rate vs post rate give you the answer? Gathering these numbers for 4chan board’s would be easy enough but I don’t know if there are enough “random” boards for a valid data set. Is there maybe another set of data points that would reveal the same trend?