However I still don't agree that it's an issue and don't see a purpose in changing the setting that we've had enabled for over a decade.
Well, the issue here is that there does not seem to be a constructive purpose to having the warning in the first place. All it does is discourage certain posts, and it's not clear what the benefit is. As it stands the overwhelming majority of threads are over 120 days old.
If nothing else the warning says "please consider starting a new topic" so if it was stopping people from responding to old threads they would be starting new topics (and that pretty clearly doesn't happen very often).
Sadly, it doesn't quite work like that. If you discourage people from doing one thing, there will be some percentage of people who will avoid doing that one thing but not do the other thing you recommend in its place. This is especially the case for new posters, who are generally less certain what about what to do and how to behave on a new forum (and the easiest thing to do, of course, is to do nothing at all and not post), In conversion rate optimization, A/B testing, etc., you notice that these are ways you lose people. Hell, just having people click one more link to do something is a way to lose people.
Just to be clear, I don't exactly expect this to make a major difference in forum activity,* but it does seem like an unnecessary discouragement to present this guideline that we do not really follow ourselves. The balance of a lack of positive effect and tendency for negative effects is why I say we're better off just removing it.
*But who knows. The forum is so low activity that just one more active poster would constitute a major difference in forum activity.