Of course the original app must also permit the receiving app permanent access to that file in the first place, plus the receiving app can only ask the OS for p

Of course the original app must also permit the receiving app permanent access to that file in the first place, plus the receiving app can only ask the OS for permanent access to a relatively limited number of files. And for files that might be opened through multiple apps (say you downloaded a picture - now you could reasonably launch a photo editor for that file directly through your browser, or through a file explorer, or your gallery or ...), each sending app will generate its own URI and the receiving app has no guaranteed way to decide whether those URIs all refer to the same app or not.