The old init will still be available, but the point is that anything built to run on systemd only, will not be easy to use with older systems without effort to backport them. This isn't much of a problem if you do choose to use systemd though, because it's backward compatible with older scripts.
It would be awfully hard to make a service that didn't work without systemd. Your software shouldn't depend on systemd, just as it shouldn't depend on init.d, rc.d, or anything else unless it's a tool specifically for working with that (like chkconfig or update-rc.d).
