As much as I'd love access to an "Old Thread" tab that has all necroposting-applicable threads available to me, I don't. Staff don't, Global Mods don't, and Administrators don't either, so we rely upon users' reports and our own exploration of threads and examination of post times/dates in order to determine if a Necropost warning is worth giving. If a necropost is unnecessary, then it ought to be warned. That said, there are such things as justifiable necroposts that contribute to a thread's initial purpose, though in nearly all cases, it's best to create a whole new discussion so that users replying to the newly necro'd thread don't feel pressured by a potential necroposting warning.