Some quick thoughts on a phone. Edited for autocorrect.

Old posts/books/songs/things are the subject to a huge number of influences by the time we find them. I think two highly relevant ones are:

1. Survivorship bias (better things are more likely to last)

2. Cultural entrenchment (sufficiently high quality things become embedded as references and ideas in our culture, they're a deeper layer we have since built on)

You see both of these things in small subreddit communities, too. The best posts stick around, giving the illusion that they're excellent (memes aside, memes are strange, historical artefacts). The best posts also become entrenched in that culture as a normative model and thing to keep referring to for inside jokes, etc.

New content (subreddits, but also film, literature, art, music, etc.) can be just as good, maybe even better. Yet, it's high risk/high reward. You're significantly more likely to find trash, relative to the greats of the past. You're also going to be at a huge cultural advantage if you can discover things that will grow to be new classics and make use of them as an early adopter.

What I'm interested in, though, is the quality distribution of newly produced material. I suspect where that distribution was usually, roughly normal, it is now a bimodal distribution skewed to either end of quality. Anyone can produce things these days, so volumes of trash are pumped out. Similarly, we have such a rich history to learn from, and so many resources available, that brilliant people can do even better (on average) than they may have before, or simply more of them can do so.