Sort any subreddit by best of all time, most posts are from >2 years ago. Reddit has been on the decline and there is no sign of that changing

I agree that it's on the decline, but that's not very good logic. Reddit's been around for over a dozen years. I'd expect most all-time posts to be from >2 years ago because you're counting top posts from the past 2 years against the top posts from over a decade prior.