File manager is a useful concept, but not essential. Btw, you must read the update too. It was reverted withing hours.
The issue with WP is not the lack of a file manager, its apps not being allowed rudimentary access to files, even user files at times.
That being said, AFAIK, the filesystem you see on a WP phone is fake in the sense that even though it appears just like another folder on your PC, it isnt. It seems to use a series of complex symlinks to 'allocate' space - space that appears to be regular directories, but arent. Maybe thats the reason why apps cnnot be allowed to access all files. Just speculating.
