The one constant of "save your favorite articles and websites offline, forever" apps seems to be that they're... very much not forever.
In my view, this not being a native, interoperable feature of web browsers is a failure of the web. I'll be able to listen to a podcast episode I've downloaded as an MP3 forever, and the same goes for ePub books (if they don't have DRM in any case) – so why is the same so hard for blog posts and articles?