XFS: the filesystem of the future?

These are orthogonal permissions. To overwrite the file you need write permission on the file. To create a new file in the same directory you need write permission on the directory. It's easily possible to have one without the other:

root# mkdir a
root# touch a/b
root# chown user a/b

user$ echo test > a/b # no error
user$ touch a/c
touch: cannot touch `a/c': Permission denied

> as for the symlink/hardlink 'issue', in my sysadmin experience, more problems are caused by editors that modify files in place (not using temp files and renaming them) than by breaking links.

Obviously that would remain an option. The point is that it should be an *option*, alongside the ability to atomically update a file in place. Symlinks are often used to add version control over configuration files (without putting the entire home / etc directory in the repository), while bind mounts are more often used with namespaces and chroot environments. Usually you want the latter to be read-only, but if the file is read/write it makes sense to allow it to be updated without breaking the link. (You can't just delete or rename over a bind target, either; it has to be unmounted.)