Instead of replace POSIX, why not expose POSIX on top FUSE and offer additional API (eject MTP drive, disconnect network drive etc.) for those very few apps that need to handle that (like file browsers)
Each app needs to implement it
Is there a simple concept like POSIX file paths that can be copy-pasted from program to program?
Can't open video file from network folder in VLC without VLC just starting without doing nothing (at least when installed from Snap), however dragging the file works (probably due to drag-n-drop not assuming recipient knows GVfs and VLC having built-int support for Samba)
The infrastructure for GVfs is just ugly, install locations etc..