It would be nice if all posts would still be linked to the main page somehow. OK, it would be nice for people pondering to crawl Hacker News. Even better would be a zipped download of all the posts...
Still, would crawling be OK? Since the old posts are not linked anymore, I consider crawling by id (check all ids up to 150000 - ugh...). I would try to minimize requests (one topic contains several ids in one go), but still...