Yeah, Bingowings is right, I think. The people on Alderaan had to have had some idea they were in peril. After all, Bail Organa was well aware what the Death Star was because of his affiliation with the Rebel Alliance. And he surely would have tried to get his people away. Also, given that there will always be space traffic coming and going, it's fair to say that a few ships must have had a chance of escape. I may try to show one getting away into hyperspace, but I'm inclined to show a total wipeout as that is what the scene is aiming to convey. Even one ship escaping undermines it dramatically, and at this point in the story that's not really desirable.