I see no reason to close topics. Personally every time I see a topic being closed I feel like censorship happened (harsh word, but really my feeling). A forum is a place for maximally asynchronous discussion. It’s not the place of quick-live chat, there is already Slack/Twitter. If I want to reply to a discussion that stopped 1 year ago, it may be because one year ago I didn’t find it interesting or I didn’t have the knowledge to understand.
Concrete example: there’s currently a very interesting discussion about Haskell and Clojure going on. I don’t really participate because I was never able to do more than hello world in Haskell whereas I’m ok with Clojure. Maybe when I finally decide to learn Haskell in one year again I’ll be happy to share my feelings over there. No need to hurry. No need to shut it down. It’s just fine that the discussion sits there. And if I feel like I should open a new topic because what I want to say is slightly off-tipic but still related I know I can. I have the choice.