Anyways saying a thread should be unpinned by a moderator is something I hardly see here to be frank. I have seen new threads pinned but replacement I haven't seen.
That is basically because no user has created a better and up-to-date replacement thread, new threads can of course be pinned if they are not related with any of the existing pinned threads, but if it is, mods/admin will replace/switch the pinned thread.
and also the creator should be ready to update it from time to time if not it's same as leaving the old thread pinned.
As long as the OP is around, i am pretty sure they would not have any problem updating the thread, but people can leave the forum for a myriad of reasons, so they could still leave the forum and become inactive like some of the OP of currently pinned topics, but that is a non-issue.