I was watching the Xiaomi phone keynote, and the introduction of a service called micloud. In a fundamental level, it looks like an amalgamation of google sync, shared photo stream and Dropbox.
I think that's what apple originally wanted to acquire Dropbox for. Something like google drive for the sharing of files.
That said, I have rarely ever felt the need for a file system. Most of the time, the app's innate sharing features more than suffice.
If I wish to share an image from a website to Instagram in Safari, I need first save it to Photo, open Instagram app, then use Instagram, then use built in file system to open the picture.
Take s other example, if I look at a PDF file in a webpage, I need to save it somewhere. I need a app to save it. If you happen to have more than one app reading PDF, then you have to remember which app contains that file.
File manager really solves all the hassle and enable you quickly complete a task. That's being said, you also need a better sharing system like Android. Otherwise, noting will make sense.