@Erik Blair I think that the important different between niches and publications is the exclusivity. The Fried Chicken Publication has more editorial control, which theoretically means better quality content...but to the exclusion of the content they DON'T publish, whereas the Fried Chicken Niche gets anything and everything fried chicken-related, giving content creators and content consumers a wider variety of options.
Additionally, as I understand it, publication owners can gatekeep readers/viewers by requiring a fee for access. Niche owners leave the doors wide open for everyone. Different advertisers will be attracted to any potential niche/publication pair because of the type of eyeballs they want to attract.
But your concerns do make sense, because we don't know enough about how publications will function to make an informed opinion.