I have to disagree, it doesn't do that at all. I still can post on all the gameplay topics from 2013 and discuss the matter there without creating a redundant additional topic.
I think it rather is there to prevent people going back and destroying basis for the discourse that happened after that post was created.
So for instance, I propose an idea and hundreds of posts are discussing it, then a year later I go and change something and it suddenly makes no sense.
Also it prevents people deleting all their posts when they suddenly get unsatisfied with the community or something ... which sadly happens a lot on forums.
It's just very annoying if you want to do stuff like this, projects that span longer then 6 months. Like this, and the minecraft server and registries of info.