Cinder will turn out to be Crazy-Prepared enough to have contingency plan for anyone trying to travel back in time to stop her...
And if she hasn't worked out that's what's happening already (let's be honest, there's only been one real incident as of this writing), she will do very soon. Future!Ruby is the Fall Maiden.
Because she killed Cinder in her timeline, and her last thoughts were directed at Ruby, but Future!Ruby doesn't know about the Maidens, with all knowledge presumably being lost at Graduation when the staff was poisoned. The Maiden's powers may have also contributed to her traveling back in time.