I don't know if they're wrong. HN's primary utility is promoting YC's job openings and product announcements alongside the more important industry-wide news. Turn it into a generic link aggregator, and owning/moderating/updating the site might be more trouble than it's worth.

You forgot to mention the great discussions that take place here and that revolve around topics that are very relevant to your readers.

Now you are talking about value but you asked about cost. They are not the same.

bigyabai is talking about utility from perspective of the owner or a hypothetical buyer, not concerned with the wider community.

It is, but that doesn't bring value for the owners on its own. Something else has to exist on top, or you'd have to run the service and moderation on donations. The forum on its own has negative value.

You failed realise that bookface is of much much more value than HN.

HN is just a free link aggregator, you can't make money from it.

bookface is only accessible to YC founders, which means YC founders have given a chunk of their company away to access it (as a bonus)

You see, the network of bookface (and bookface its self as a part of YC) is more valuable than HN.