Recognising that HN discussion will go where HN discussion wants, I'll point out that my principle point was that suggesting "subscribe to sources" simply does not scale for HN, or any site or service with a similar dynamic (Digg, Reddit, Mastodon, FB, etc.), where stories are submitted from a large number of online sources.
Answering your question quantitatively: Whaly did an analysis based on the HN API about a year ago in which they tallied all HN story submissions. Comparing that value against the number of front-page stories in my own dataset, roughly 3% of submissions reach the front page. I could also determine that front-page posts see just under half of all comment activity. See: <https://whaly.io/posts/hacker-news-2021-retrospective>
As anyone who's submitted some number of stories to the queue knows, the HN queue is a capricious beast, and success varies tremendously. That said, it's also a somewhat subdued beast, and there are influences over it, including automated and explicit moderator actions, member flags, etc.
In other discussions I've commented at more length on how the front page archive is a mix of numerous influences. Looking over my own front-page items, and comparing them both against the larger set of what I've submitted as well as my own preferences and biases, what's landed really doesn't match my intentions particularly well. In that sense, the front page is a dance between submitters, other members, and the HN site / YC policy itself. There was some good discussion of FP characteristics and caveats by itunpredictable here: <https://randomshit.dev/posts/what-gets-to-the-front-page-of-...> (HN discussion: <https://news.ycombinator.com/item?id=36521887>). TL;DR: the front page (and its archive) are a view of HN, but not the only view.
Anekāntavāda.
Dang has mentioned that submission activity to HN has been reasonably stable over time, going back to 2012 (at least as of four years ago): <https://news.ycombinator.com/item?id=19214098>
I've looked at stats on how many votes are required to make the front page, based on the 30th (lowest-ranking) story position, and how that's changed over time, here: <https://news.ycombinator.com/item?id=36670430>
(Original author)