With file access, the sensible solution would have been of course to just overlay the permissions onto the regular files API (like I think every other sandboxing solution does), so everything that uses files could keep working as usual and you'd just have to add some limited code to request the appropriate permissions [1], but I guess that would have been too sensible, and when all you've got is a SAF, everything looks like a nail or something…

From what I've gathered, getting access to a single user-picked file via SAF isn't too painful, but for browsing a whole directory tree it's more annoying having to use a completely new API for that, doubly so when you're using a library that expects regular files, plus performance is somewhat lacking, too (judging from complaints and bug reports I've seen, both in that it is easy to run into performance pitfalls, but also that even best-case performance is still worse than classic file access).

[1] I think something like that was actually implemented for Android 11 onwards, judging from https://source.android.com/docs/core/storage/scoped#using-sc...? Although possibly still at some noticeable performance cost.