You know what I don't understand!?
Thread necromancy is not a new thing, its almost as old as the internet itself. So why, oh why doesn't forum software these days come with anti-necro functionalities?
Something like; When a thread has been inactive for more than x amount of time, it goes in archive mode. When viewing an archived thread, there are visual cues in the layout indicating it is an archived thread. When you hits the reply button in an archived thread, you get an unavoidable warning saying something like: "you are about to reply to an old thread, are you sure you want to do that?"
This could even be done on a per post basis, for example, when replying directly to an old post in a still active but very long thread.
Seems like a gap in the forum software marked to me.