So basically they just threw away the entire playable cast from XIII
That's too early to assume. While Lightning has lesser playable times, she is definitely playable as seen in the demo. Who knows, snow, sazh and hope can also be playable at some points in time.
However, if they really throw away the entire cast, I myself don't mind that. I actually wanted to take this game as a new game with new features, with some allusions to the previous game. I am so hyped up about the additions, especially the interactive towns, possible time-travelling/dimensions plot and explorability.