The major problem isn't replacing the blanket READ_EXTERNAL_STORAGE permission with something a little more fine-grained, but the fact that after requesting access to some user-picked directory tree, apps still have to go through the Storage Access Framework with all its warts and deficiencies instead of being able to use normal file system access if the picked directory is part of the local storage.

If it had been implemented that way, there still would have been some usability problems (some users will likely always manage to pick the wrong directory and then wonder why the app doesn't work), but other than that it would have caused much less trouble.