> Critically, apps can not directly read your files, they can show a button which will open the OS file picker and the user can pick a file to open which then provides access to the app.

Unfortunately this model breaks down for any kind of file type that doesn't simply consist of a single, atomic file, e.g. various constellations of media files (playlists, subtitles, multi-part videos), multi-part archives, HTML files, georeferenced imagery with sidecar files, etc. etc.