A core part of browsing Hacker News or Reddit is opening the links and reading the articles, which are web pages. Once I'm opening web pages, then I want to be able to bookmark them, arrange them in tabs, find them in my history, have them saved in sessions by my session manager extension, configure how they are handled by my ad blocker, search for related information, and so on. In other words, I want a web browser.
The same issue applies the other way around, too. When I follow a link _to_ Hacker News, I want it to show up consistently. I don't want to end up with some Hacker News pages in browser tabs and some in a separate app, and then have trouble remembering where to find that page I was looking at yesterday.
I get that it can be nice to have a slimmed-down UI for specific purposes. But I'm puzzled by this particular use case because there is no neatly contained navigation sandbox — as soon as you follow a couple of links, you're just browsing the Web.
So why won't there be an inexorable push to expand Yack's feature set until it is a browser? Why won't users eventually switch back to using their regular browsers?