I'm not sure this is such a good thing. The walled-garden approach does have some benefits but it gives up lots of control by content creators and creates an even stronger network effect and disadvantage to everyone else, as you pointed out.
A big issue seems to be that lots of publishers just haven't been able to get their mobile (app and website) experience to an acceptable level, either through a lack of talent, experience, understanding of digital or some other combination.
Facebook’s main argument is that content should load faster, and that’s perfectly valid and important, although I'm not sure why they claim a drop in app usage because of it. Almost all links from the mobile app just open an in-app Safari window rather than the full Safari browser app, which means closing the tab puts the user right back in the Facebook app. There’s no loss there.
The content itself is also physically the same, text weighs no less on Facebook than it would elsewhere. The fault is really with publishers who have, by working with bloated ad providers and basically adding tons of weight and extraneous “media” to a page that nobody ever reads, pushed themselves into this situation.