Tell that to newsstand and stocks... Finder for ios would get more use, guaranteed.
I know tons of people who use both of those. Granted they are older, but nonetheless put those apps to good use.
Might I also point out a flaw in your argument. Stocks and newstand, being cloud based apps, you can't accidentally delete files forever. A finder app on iOS, some people might get into it and not know what they are doing. They could accidentally permanently delete files they need. Or, a more likely situation, they could be using an app to edit a document. Then in Finder see a document named 3278-157ah37b (or some name they don't recognize) and not know what it is so delete it. Then once they open that app they used to edit the document see an error saying it can't find the file.
I know plenty of people who have had this happen to them on Android. They need to free up space so they think they will download a free file browser and delete the stuff they don't use. Except the problem is they don't know what files they use or don't use because the apps that use those files don't explicitly say. So they end up deleting files they want to keep. It may never happen to you or to me, but we do not represent the average consumer. And this has happened many times to people I consider average consumers.
Having a completely open file system that is shared between all apps paints a pretty picture. But it opens up huge problems to the average consumer that can ultimately hurt the user experience.
Convince me how accidentally deleting files you need brings a good user experience and I will say you are right. Good luck.