I can also have Debian Jessie without Systemd init. But I can't have it without Systemd, the middleware or "2nd kernel" or however you may call it. (Actually I can, but what remains has little to do with what Debian is about.)
That's why I said it's much more than an init system. And most of the discussion totally misses the point because it only revolves around the (comparably insignificant) init aspect.

btw: This is the point where I think this topic should be split.