I don't think you understand what memory-mapping a file does. On a SATA device, it allocates some ram and copies the content of the device into that ram when accessed. On any device that allows direct random access into it (such as NOR flash currently), memory mapping just attaches that memory at that point in your address space. When you memory map a NOR (or XPoint in the future) page into memory and access it, there is no translation of access beyond the one that is always done for RAM, it's just a direct bus access into that device.
It's not done this way for current PC and smartphones though.
As I said before, the only thing that the visible VFS layer of filesystems provide is a translation of human readable paths into pointers in devices. All the other features of filesystems are completely orthogonal to this. I never see this system going away, it's just too practical.
I'm not saying current system should be going away (as I said, we'll likely see current system going on external storage in foreseeable future). I'm just saying for some part of the system maybe we can use something different.
I'll use another example. Right now we have a mapping system inside an SSD for wear leveling (we'll still need that even with the 3D XPoint memory). With some modification we can make each file in the SSD looks continuous, and that would eliminate the need for nodes and bitmaps in a traditional file system, because in theory the controller is already doing that.