I don't think it's necessary to automatically lock down a discussion just because it's old. I have no problem with someone finding an old thread on a topic and adding a new post to it because they found something new that's relevant and addresses an unresolved issue in the post, or shows the latest progress in an area of research. In fact, in that situation, I would prefer they post in the old thread rather than create a new one without the benefit of the previous discussion; this both gives perspective to the issue and saves people who were previously involved in the discussion the trouble of having to rehash the same points again.
I can think of three situations where necroposting is more annoying and perhaps inappropriate. First, the obvious situation where a thread that pre-dates existing rules gets resurrected with a lot of violations of the current rules. In that case, yes, a lock and split of the new post to a new thread seems fitting, to make it clear that the old thread should be read with caution and doesn't fit current rules. Second, when someone has asked a question that is of only short-lived interest and has become moot. For example, if someone asked for advice about particular university choices based on specific information about their academic or personal background or interests, there is no point in replying to the thread perhaps even 6 months later, and certainly not by a year or more later when they would have already made their decision and been enrolled in university...definitely not 4 or 5 years later when they are already graduating from that university.
The third situation is when someone replies to an old thread with a post that is only very superficially relevant to the original topic, or with something that adds nothing to the old discussion. For example, responding with, "Hey, that's cool, thanks!" is pointless and not worth reviving an old topic. The other part of this type of situation is really more of an off-topic post problem than just a necroposting one.