It is driven primarily by the size of our fingers and hands. Our hands can only hold devices of a certain size (and our bags and pockets also put practical limits there). So unless we change human physiology, touch remain limited in this manner.
> Just consider an interface that presents 10 000 files
> as a 4 level hierarchy
Sounds horrific; but also (thankfully) utterly unrealistic for the common use case for tablets. So instead, lets Consider 10k files but organized by metadata, activity and tags. Most of that information is automatically generated.
This allows us to avoid the file system hierarchy, lets files appear in more that one "area" of the metadata space AND lets us get to them quickly by defining sets of tags.
In the file manager you can go to the "Tags" sidebar and select as many tags as you wish. So instead of a 4 level hierarchy, we would have an N combination tag set.
Perhaps a good example is portable (and other) music players: good ones let one quickly go through the content by artist, album, genre, name, etc. as well as custom playlists. Where are they on disk? Doesn't matter.
This brings that same concept to all files. Due to the flexibility of tags and metadata it is far more efficient. Metadata from e.g. the camera that took a picture or a location tag can be used to automatically group. Tags are easy to create and use. There's also a (zoomable!) timeline included.
So you could have one document tagged with "peer review", "PLOS" and "Neurobiology" (to pick sth rather at random). Then you can see that document, along with its peers, by selecting any combination of those tags. You might start with "peer review" and then go to "Neurobiology". you might start with "Neurobiology" and then decide you want to narrow it down to only peer reviewed papers.
Honestly, it kicks the living crap out of single entry hierarchies.
> search does not work well if one is unsure about the name
which is why search includes more than just the name. welcome to the 21st century in terms of information storage and retrieval :)
> grouping allows quickly see neighboring groups/folders
tags are groups that are more flexible than a static hierarchy.