At this point, Nautilus really is just a file manager! It isn't the file chooser; it doesn't mount external devices; it isn't the desktop background.
If something is hindering one's ability to create an alternative file manager, there is probably a completely uncontroversial patch that can be sent upstream.
If something is hindering one's ability to create an alternative file manager, there is probably a completely uncontroversial patch that can be sent upstream.