If something is actually relevant, someone can click 'report post' and ask a mod to unlock the topic. (Or start a fresh thread to ask about it, and link back to the old thread)
It's fairly rare, but we have had people come back months/years later to update a forgotten thread they started or were talking on to share a final resolution or workaround. In that case, one of us unlocks the thread and they can post in it no problem…
Occasionally we'll go back and edit a locked thread if it turns up high in Google searches for a particular topic as well, to lead people to better/more accurate info. In those cases if the thread were unlocked it would probably get a lot of irrelevant posts that would be better served by new threads.