I totally agree that the metric is imperfect for a long term analysis. I was initially leaning toward a quantile based approach to really focus in on topic trends over time, but when I was initially exploring the data, the relative challenge of having a Show HN become popular in 2025 compared to previous years caught my curiosity, and for this decade I felt a static cutoff provided a simple and easy to understand threshold.

I do think as a metric for total reach, a static cutoff actually works reasonably well. I think some form of square root normalization over total users is probably the best balance.