Written Before the Ending
A time capsule is written for a reader who doesn't exist yet. Whoever buries it already knows an ending is coming, the date on the lid says so, and everything inside gets chosen with that future reader in mind. It's a performance of a moment, aimed at people who weren't there for it.
Most comment sections aren't performances of anything. Someone typing "day one, cannot wait" under a trailer, or "just backed this, going to change everything" under a crowdfunding pitch, wasn't writing for you. They were writing for the five other people scrolling that thread an hour later, and possibly for nobody. Nothing about the text anticipates being read after the show gets cancelled or the company folds. That's exactly what makes it worth reading after the show gets cancelled or the company folds.
The difference is what each writer knew about their own future. The time capsule writer knows an ending exists, even without knowing what it is, and that knowledge shapes everything they put in, cleaned up, self-conscious, written to represent the moment rather than just be it. The comment section writer doesn't know an ending is coming at all. As far as they can tell, the story is still open, because it is. That certainty is the whole value of the artifact. You can't fake not knowing something.
Go back far enough into any thread that outlived its subject and you find people mid-anticipation, arguing about release dates, certain they're early rather than wrong. Nobody flagged the page for preservation. Nobody decided it was worth keeping as-is. It just wasn't deleted, which on the internet turns out to be close enough to preservation to count.
That's the part that gets me: the honesty here is a side effect of neglect, not a design choice. If a platform had decided a thread like this mattered and archived it on purpose, curators would have shown up eventually. Someone would have added context, a note explaining what happened next, a warning label at the top. The record would start talking to the future instead of just sitting in the past. The moment it does that, it stops being evidence and starts being commentary.
The threads that survive by accident don't have that problem, because nobody thought they were worth the effort of curating. They just kept existing under a video or a post the platform never got around to pruning, picking up a stray reply every year or two from someone who found it late, until eventually the ratio flips: fewer people excited about what's coming, more people who already know what came. The two audiences share a page and never speak to each other, separated by nothing but the order the comments happen to load in.
I don't think you can build this on purpose. The moment you try to preserve pre-ending anticipation for later reading, you've told the writer an ending exists, and that's the one thing the format can't survive. It only works written before the ending, by someone with no idea one's coming.