I highly recommend to read this great evaluation of upstart and systemd and the discussion around it.

For me as an upstream who wants to integrate with systemd it's valuable information. My idea so far is that a Plasma on Wayland shell needs to be brought up by systemd. Especially I consider using socket activation to start the KWin session compositor. That's something I want to discuss in detail with +Àlex Fiestas at the next Plasma sprint and try to prototype the unit files for it. My assumption here is that any system which can run Wayland also uses or provides systemd (BSDs don't run Wayland). As the Wayland shell is new technology depending on new technology I so far do not see a compelling reason to not rely on the awesome technology provided by systemd or to provide multiple implementations.

Of course a legacy startup will still be around to bring up Plasma on X11. So for the BSDs nothing would change. They can start Plasma on X11 without systemd. But everybody on Linux would benefit from using systemd and Wayland.

What about Ubuntu and everybody else who might consider systemd as the tool of evil? Well there is nothing to stop Ubuntu from shipping systemd and to a large part they are already doing it. Whether the init system is used should not be relevant to us.