As well as upower (though it still works with ConsoleKit as fallback, I think), udisks2, the Weston compositor and some other programs. Binary distros also have some pretty skewed dependency chains on systemd for packages where it's inessential.
Only up to a point. Systemd is getting too bloated, and software engineering tells us that the effort to maintain this will grow exponentially.
So we have two options: if systemd improves and decides to follow UNIX principles, good. Otherwise it'll be replaced like consolekit et al. It's great that we have so many init systems to choose from. The only problem now is software that hard depends on systemd when a single line of code could avoid that.
It'll only grow, though. Most certainly.