Nishimura's saying something a bit different (tweet here): The background art was too big to store even at Sega's offices, so most of it was disposed of. Looking back on it, he wishes he could have saved these. He thinks that if he had left the animation cels to Sega to take care of, the possibility that they would not have been saved is high. He doesn't say anything about the cels from the photo deteriorating. He's just speaking in general: if you don't take proper care with how they're stored, they will deteriorate. In other tweets, someone asked him if he would scan them. He replied by saying they have value because they are physical objects (or their value lies in them being physical objects). Basically, I don't think he'll scan them.