There are legitimate reasons to reply to an old thread in my view. Maybe you just got a crash log for a rare, unresolved bug. Maybe you see how someone’s technical writeup for implementing boats from 2013 would mesh perfectly with an idea you just had. Maybe you just want to thank a content creator for their old video or mod you just enjoyed.
The important criteria are A) Is the content of the thread still relevant today? And B) Do you have something to add? Most threads probably don’t meet A), but if we lock all old threads, we’ll throw out the good with the bad. The bad includes multiple threads on the same topic, which is disorganized, and collectively forgetting important information simply because it’s old and gets buried.
And for what? Because we’re insecure about the forum feeling ‘dead’? I don’t think so. The best outcome would be for everyone to think about whether they’re gravedigging and what makes that bad before they post. Failing that, I don’t see the problem with reporting gravedigging to the moderators case by case.