I assume HN readership has grown, with readers probably giving out roughly on average the same number of up-votes they used to for story submissions per any tim

I assume HN readership has grown, with readers probably giving out roughly on average the same number of up-votes they used to for story submissions per any time unit (maybe some variability). The number of points it takes for a story to beat out sibling stories should then also increase. There may be other effects such as a wider-variety-of-stories-being-submitted than from before that would perhaps squash per-story-votes-needed-to-stay-on-front-page. One might say "but there are more stories being submitted too" -- but the unique number of these wouldn't go up linearly as the number of readers goes up, since they're pulling from roughly the same universe of possible stories-of-interest. EDIT: paragraph about "more stories submitted"