I really don't get why some people think "necroposting" is somehow a bad thing. O.o
If you ask me, threads should stay open indefinitely, unless there are specific and good reasons for deviating from that and closing it. For instance, topic threads that have become so long that it simply makes sense to start a new one and keep the old for reference (not that I have seen that sort of thing here anyway).
You may think these kinds of posts are spam, but that doesn't mean they are spam. I'm also not sure what your point is about having seen posts being locked in other forums for "necroposting". It just sounds like an example of a non-sensical policy and volunteer moderators having to waste their time on something that is detrimental anyway.
Spam is spam. Conversations aren't. If anything, what is lacking is the ability to sort forum activity feed by thread recency (+ latest reply) rather than advocating for policies that handicap the fundamental purpose of a forum as a workaround to something that is much easier fixed by simply providing simple sorting tools. That way, everyone gets what they want. Instead of forcing a preference on everyone.