> people have been saying that the quality of HN has been falling ever since HN started 15 years ago
I agree with them. I’ve been sticking with it for these 15 years, but I’m sure you’re aware of a certain site that does a pretty good job characterizing how the comments on many of the most popular stories have devolved.
> Internet forums are super weird and there are limits to what one can do about it. I wish it weren't so.
Respectfully, this is a cop-out. This is your site. You can do anything you want to make it fit your vision, from making it members-only, to making every post subject to review. I’m not trying to tell you how to run your business or anything, but to throw your hands up in the air and say (paraphrasing), “Internet forums are hard, what can you do” strikes me as forfeiting responsibility.
> different users have completely contradictory images of what that would involve,
It doesn’t matter what users think. This isn’t a democracy. This site belongs to you and its owners. How it’s run, what the rules are, and who can be a member are choices you exclusively get to make. You are responsible for its tone and character. Don’t burden the members with this responsibility without giving them the direct ability to enforce it.