For example, the apps from Omni do this, as do obsidian, Linea…
Let’s assign the blame where it should be here.
Obviously the blame lies on Apple for locking away your device's contents from you. Developers should not be able to have more control over what you can access on your device than you do. Even if they make bad choices (like making accessing the files hard) it should be you who has the final say, not them.
Apple making it possible for developers to make bad choices and go against users' control over their own devices is to blame.
If you chose to use iAnything then it's a bit late to start complaining about lock in now.
However, sure, lots of users chose Apple knowing exactly what it is. Apple's not going to change since their model clearly appeals to lots of people.
If you don't like Apple's model, then don't choose Apple devices. What everyone else chooses is somewhat irrelevant to you. (Other than network effects noted earlier.)
They are hardly irrelevant, especially if you like money.
I can hack up a "device" with a raspberry pi zero or whatever and call it "HaxyDeck" and claim it is all open to anyone who wants to tinker with it, but at the end it'd be irrelevant because only me (and perhaps a couple other people) would have it. The aspects you want to ignore (number of users, be something other than Apple, what others are using) would actually affect my use of HaxyDeck directly: since i'd be the only one (or one among a tiny number) using it, i'd be the only one having to make it do things i want, it wont have software from others, it wont support software other people may want to use for communication, software some services that theoretically have nothing to do with phones or computers (e.g. banks) wont work because HaxyDeck's userbase is irrelevant for them, etc. All of these have to do exactly with what others are doing.
Basically see how all the non-Android Linux phones (like PinePhone) are faring. You can't just ignore what effect having a large user base some platform (be it a device, an OS or even service) has and say "just use something different".
It's not the 2011 iOS anymore, if an app today hides its video projects from the user, it's entirely the app fault.
I imagine, for example, that if the internal project files for a popular video editing app were accessible, we’d see competing and/or open source apps emerge that could parse them, were the original app to become suddenly unavailable. Instead they’re just lost because your phone won’t let you access them.
while possible, it is fairly non-trivial for iOS apps to have read/write access to a shared folder where they can drop arbitrary files, which can then be accessed by other apps, or be discovered by the user. it often requires copious permission negotiation handling codepaths by the developer, and a fearlessness of scary permission-warning dialogs by the end-user.
even on modern (commercially popular flavors of) Android which no longer imbibes the "free software" ethos of the linux core the OS was built around, you can't access formerly accessible application sandbox folders without installing third party browsing tools or plugging into a desktop computer to mount the storage, and cross-application sandbox access is similar to iOS.
in the "personal computing way" mentioned by the article (even today on desktop environments, less so on MacOS) program outputs are default-visible, and developers have to go out of their way to firewall or obscure or encrypt it from being accessible by the user or other programs using OS-provided pathways.
i think this is 100% on the OS + hardware + application platform provider (with Apple as all three on iOS).