If the point of HN is sharing information, and having substantive dialog, then these links are counter-productive. If really think we should ban non-traversable links; if there are alternative links to submit that are the next best available summary or secondary source, I think that would be far preferable (and we can always relegate the primary paywall-ed source into the comment page).
Should I continue to "flag" paywall-only links as I see them, or just continue to gripe in the comments?
I don't know of a robust way to replace paywalled links with open ones to the same story. Sometimes people post Google search links to WSJ articles, but those don't work for everyone, and presumably there would be legal questions even if it did work technically.
It's also not as simple as the publications not wanting anybody other than subscribers to read their content, since they choose how high to make their paywall, and most have well-known workarounds. WSJ is one, though theirs is a step more annoying than NYT et al.
Previous comments on this:
https://news.ycombinator.com/item?id=8777971
https://news.ycombinator.com/item?id=8634076
https://news.ycombinator.com/item?id=8647853