Well, this post and http://lwn.net/Articles/616571/ does seem to indicate they're planning to keep some level of compatibility with other init systems at least.

I'm not in love with sysvinit, I just don't think systemd is the best option. It clashes, at least with my views on simplistic system design philosophy. OpenRC and FreeBSD's rc seem notably better than both systemd and sysvinit to me.