systemd is an offer. You don't have to take it. You can continue to maintain CK and all the other stuff if you want. Nobody stops you. I am currently doing most of the maintenance work for CK but I am happy to pass it on to a capable person who's into maintaining legacy software.
I am kinda allergic of allegations we'd force things down on anybody's throat by integrating things. That's impossible in a free software world and kinda insulting too, because we make you a free offer, and it's completely up to you to take it or not.
It is much easier for us to provide these things in systemd, since it allows us to bind seat info to the session info we already maintain for users in cgroups. Doing these things in code we ship with the same package his hence really simple and requires very little code. (Note that all this seat/session management is done in an auxiliary service -- a process with a PID != 1. You can strip it out easily (and you absolutely want to do that for embedded uses, actually).
If you keep things separate just to keep things separate you add a lot of work and a lot of duplicate code and make it very difficult actually doing the perfect 1:1:1:1 linking of the user cgroup, the XDG_RUNTIME_DIR, the user systemd and the user bus. Also, what benefit would it bring us (the developers) to keep things separate? It's a simple fact that the folks who'd benefit from that (i.e. the Upstart fans) wouldn't say "Thank you" anyway (they'd much more likely find something else to bitch about), so why should we bother?
You know, tieing this to systemd makes things a lot easier for us and the end product much simpler. That's the reason why we are doing this. While I actually do believe that systemd is the one and only init system (hey, I can be proud about the stuff I wrote can't I?) using that as the the only reason, or a reason at all would be pretty bad technical decision making.
Also, we are not the only ones who can write code. If you think you can do it better and have better ideas, then do it better, but don't complain if we try to bring Linux forward.