Looks like we're still in disagreement about this. As far as I can tell, there was never a 'hard' dependency of GNOME 3.8 on logind; logind was an optional dependency[0].
> beyond functionality that can be summarised as "SysV initd plus dependencies"
I would attribute that to the lack of similar "service manager" functionality in other "boot managers". That comes down to the hesitancy of these "boot managers" to embrace the role of "service managers". There's tons of functionality that the systemd project (not necessarily the pid0 binary) introduced to make services run consistently and reliably. They paved the way in that regard and we're still waiting for contenders to catch up.
[0] https://mail.gnome.org/archives/desktop-devel-list/2013-Marc...
As for supporting configuration format - it's an obvious move, though more as "import" rather than only source of truth (different internal data models), and the format... is not exactly good (mostly due to lack of sane defaults that start to hit you once you wander beyond functionality that can be summarised as "SysV initd plus dependencies")