That's why, just as fork(2) is a primitive for the process creation, copy(2) should've been the primitive for the file creation — creates an exact copy of the file under a new name, but with the exact same content and all of the metadata (except for the name, obviously), including its kind, permissions, timestamps, etc. And no, it wouldn't be prohibitively expensive because all filesystems can quite easily support CoW; after all, most of the created files will be truncate(2)d almost immediately, so there is no point to eagerly duplicate the file contents.
The "metadata is atomically copied" part would support very nicely the usual text editor's idiom of rename(2)ing a temporary file over the source after fully writing it out — you still need to accurately replicate the permissions and extended attributes. And just as shells are important enough programs to have fork(2) almost exactly suited for them, it would make sense to have copy(2), suited for the text editors.
The "metadata is atomically copied" part would support very nicely the usual text editor's idiom of rename(2)ing a temporary file over the source after fully writing it out — you still need to accurately replicate the permissions and extended attributes. And just as shells are important enough programs to have fork(2) almost exactly suited for them, it would make sense to have copy(2), suited for the text editors.