Hmmm...interesting idea. (But I think he means posts per page?)
Most threads do indeed have a life cycle. As people say everything they have to say on a topic, it tends to dissolve into banter and/or stop getting new posts....and yeah, this often happens around page 2 or so of the thread.
I've always assumed that this happened largely due to a simple lack of new ideas on a topic...everyone has pretty much said their peace and moved on.
Do you think more posts per page would extend the conversations?