I don't understand why systemd is suddenly such a hard requirement: it suggests that the Gnome developers are simply punching through the abstraction layer provided by dbus for reasons of convenience. If some part of Gnome needs a system service, it requests it via dbus and parses the response. How that service is delivered ought to be irrelevant.
Since systemd is supposed to offer all its services via dbus, is the problem really "we don't want to implement all these services that a modern desktop needs". OpenBSD et al could offer a dbus layer that implements the same set of services, but they don't want (or have the manpower) to do so. In which case, I'm not really sure they have a leg to stand on: the things systemd offers really are important for a modern well integrated desktop (or any other system for that matter) and they weren't well served by initscripts. OpenBSD is free to implement a service that offers the same dbus interfaces whenever they want, but complaining about Linuxisms is perhaps a little rich: if you want your system to remain stuck in the early 1990s then that's fine, but you can't really expect the rest of the world to hang around waiting for you to catch up.
NB If the answer to "why can't you write dbus interfaces" is "we'd love to, but the Gnome developers keep changing the interfaces every minor point release" then that's probably a valid retort...
Since systemd is supposed to offer all its services via dbus, is the problem really "we don't want to implement all these services that a modern desktop needs". OpenBSD et al could offer a dbus layer that implements the same set of services, but they don't want (or have the manpower) to do so. In which case, I'm not really sure they have a leg to stand on: the things systemd offers really are important for a modern well integrated desktop (or any other system for that matter) and they weren't well served by initscripts. OpenBSD is free to implement a service that offers the same dbus interfaces whenever they want, but complaining about Linuxisms is perhaps a little rich: if you want your system to remain stuck in the early 1990s then that's fine, but you can't really expect the rest of the world to hang around waiting for you to catch up.