I think it's a valid question.
I used to post on a forum that had the exact same format code as this one (totally different and unrelated site though and a much larger and more active forum, too...but full of nasty people unfortunately as well.) It had many topics, from regional to subject matter focused, and unlike this forum (I believe) it indexed to Google, so threads there would come up in google searches if the right keywords were entered. I posted there for years. Like 7 or 8 years, I would guess.
The policy at that site was that thread consolidation was encouraged. So it was encouraged to search for your topic and post to existing threads over starting new ones, and mods would even merge related topic threads into one.
I am not sure that such an absolute stance is sensible...but I'd advise the following as what I'd prefer to see, if I were writing the rules (which I'm totally not, because I'm not a mod...just sharing my $0.02.)
I'd suggest that posters can post to older threads if they add to the post, at the beginning of the post, a tagline like "NEW POST - OLD TOPIC" so that people are alerted to that, and if they are speaking to the subject as opposed to, say, giving input or advice to the original poster (who, even if they're still around, has likely found a solution to their issue by now.)
Alternately they might be encouraged to start a new thread and link the old thread as a reference.
I do hate it when I read a number of earlier posts only to realize it's years old...so I'd like that it at least be brought to my attention. But there is sometimes valuable content on a given subject in an older thread and it's nice to fish it back up....as long as the new poster isn't speaking as though the old events happened yesterday and giving advice that is no longer relevent, and THAT is the kind of necro-posting I'd discourage.