The basic use case works, but quite a few things beyond that are broken. None of the various sandboxing attempts (not just Android, but everything) properly handles multi-file file formats [1], so that alone is very much not a solved problem!.
Keeping an LRU list for files that were opened from outside of your own app becomes needlessly complicated [2].
Access gained that way isn't directly compatible with things expecting classic File API access (and sometimes those things are outside of your immediate control, like external libraries and even parts of the Android framework itself), and unfortunately the easiest workaround to that problem is just copying the file into your own private storage.
[1] As far as I'm aware, only macOS attempts to at least handle related files that only differ in their file extension, but even that still doesn't cover more complex file formats like playlists, or HTML or DWG files or whatnot that can reference arbitrary other files.
[2] If the same file is shared via different apps, the way things have been implemented on Android it becomes more complicated for the receiving to check whether those two incoming file shares actually refer to the same underlying file or not. Plus for an LRU list to make sense you need to try persisting the file permissions so you can still access the file later on [3] and also especially take care not to leak those permissions when you clean up the LRU list.
[3] Which also leads to some strange scenarios like when you switch to a new file manager and uninstall the previous app, all LRU entries in other apps that were originally opened via that previous file manager now suddenly become invalid.
Keeping an LRU list for files that were opened from outside of your own app becomes needlessly complicated [2].
Access gained that way isn't directly compatible with things expecting classic File API access (and sometimes those things are outside of your immediate control, like external libraries and even parts of the Android framework itself), and unfortunately the easiest workaround to that problem is just copying the file into your own private storage.
[1] As far as I'm aware, only macOS attempts to at least handle related files that only differ in their file extension, but even that still doesn't cover more complex file formats like playlists, or HTML or DWG files or whatnot that can reference arbitrary other files.
[2] If the same file is shared via different apps, the way things have been implemented on Android it becomes more complicated for the receiving to check whether those two incoming file shares actually refer to the same underlying file or not. Plus for an LRU list to make sense you need to try persisting the file permissions so you can still access the file later on [3] and also especially take care not to leak those permissions when you clean up the LRU list.
[3] Which also leads to some strange scenarios like when you switch to a new file manager and uninstall the previous app, all LRU entries in other apps that were originally opened via that previous file manager now suddenly become invalid.