The main issue is the (sometimes) hard signup wall. I've been a moderator on HN for longer than 404media has existed, and I know from experience that this changes from time to time or article to article. Other paywalled sites that appear on HN (WSJ, NYT etc) have a porous paywall; you can (almost) always get around it by using an archive site like Archive.today.
If it's a good article (contains significant new information and can be a topic of curious conversation) and a paywall workaround works for that article, we'll happily allow it.
Since HN doesn't really facilitate any workarounds anyway and we've been doing manual archive links and content reposting as needed in other cases... I suspect we can handle 404 as well as a community.
Even porous paywalls can have a marked effect on story performance on HN.
The New York Times tightened its paywall markedly in August 2019, with a net effect that appearances in the top-30 stories on HN's front-page archive (the "Past" links in the site header) fell to ~25% of their previous level.
I'd asked dang at the time if HN had changed any of its own processes at the time. Apparently not.
I suspect then that this reflects frustrations and/or inability to access posted articles behind the paywall.
If it's a good article (contains significant new information and can be a topic of curious conversation) and a paywall workaround works for that article, we'll happily allow it.