I honestly think it would have been a terrible idea to kill her off. In my opinion, by the time of either S5 or S6, the Mane Six are really starting to elongate their character arcs, especially when there are other characters that could use the screentime. With Starlight, this has become more possible as she formed a group of friends that hardly got any screentime before she was introduced. Ex. Trixie, Maud Pie, The Changedlings, even characters like Doctor Whooves and Octavia got a decent role thanks to her.