I think perhaps you miss my point:
If libijs is maintained for Debian as part of the GPL Ghostscript
project, and we also (in the future) maintain GNU Ghostscript, then GNU
Ghostscript will need to depend on GPL Ghostscript, making it impossible
to ship a release of Debian with only GNU Ghostscript, not GPL Ghostscript.
If, on the other hand, Debian maintain libijs as a separate project,
then a release of Debian (or a derivative thereof) can choose to ship
with GNU Ghostscript and not GPL Ghostscript.
The question is not whether upstream development happens as part of GPL
Ghostscript or not, but whether we should maintain a library separately
which is _usable_ seaparately.