Agree strongly with your first factor. Early adopters often (though not always) self-select for higher-level interaction. This has been true of media dating back to cuneiform.

A third factor addressing 21st century media rot is the absolute dependence on advertising, and the demonitisation of earlier ad-supported formats (print newspapers, print magazines, radio, television, and increasingly non-social-media online platforms) as ads transferred first to "new media" (itself an old story, though dating somewhat more recently to the origins of mass-advertising in the mid-19th century), and ultimately to "adtech", with highly-personalised, highly-targeted advertising.

That latter both led advertisers to abandon non-targetable media (including generic online banner ads), and the increasing pandering of online media (especially Search and Social) to advertiser interests. This both killed ads-as-financial-foundation of other online media, and increasingly turned Search and Social into ad-delivery rather than information-delivery / community-connection platforms.

It's the odd standouts which have independent funding which are still relatively immune to this. HN is one of those, some sites such as Metafilter and the Metaverse are others online. Public media would be another notable exception, though of course it is being increasingly targeted as well, largely for political reasons, notably in the US (NPR, PBS, and the late CPB), and UK (BBC). It's healthier elsewhere, notably in my experience, Germany.