It's different than awk because the decision the admin is making ("which
init system do I want to run"?) isn't done through alternatives. So you
can't use the alternatives system to coordinate swapping all the
different bits together.
It seems retty reasonable to me that the systemd maintainers don't want
to support systems which are running arbitrary combinations of systemd
with alternatives to some parts. And that a package with a Depends on a
particular systemd version should be able to depend on features from
that version; alternatives would allow an old version of opensysusers to
be used instead (unless systemd kept adding conflicts against
opensysusers whenever they add a new feature, something I doubt anyone
would be happy with).
Strikes me as there is a possible solution, though: have opensysusers
dpkg-divert it and put a shell script in its place that checks which
init system is running, and exec's the right sysusers based on that.
This wouldn't affect systemd-only machines (as opensysusers would not be
installed at all), and would do the right thing if someone has installed
two init systems to, e.g., consider switching. It'd need to be a script
that the systemd maintainers feel reasonably confident will always run
systemd's implementation when systemd is running, to avoid the mixed
implementations issue.
(Of course, there is the problem with temporarily having no file
there... but I suspect that could be documented around as the lesser of
evils. Especially if done with --no-rename, so it'd be a very short time.)
