I always wondered how effective SA was. With it enabled, there was still decent accuracy. Maybe 50ft. Certainly accurate enough for a missile targeting system. So maybe the conclusion from the government was why bother. However I do believe that there is more to SA - a simple encryption algorithm could disable the system completely.
Also, where are the other satellites positioned? Satellite positioning is important. Also, the number if satellites affects accuracy.
Also the system runs software which allows for updates and modifications. Easy to reconfigure the individual satellites and system. There is constant communication with the constellation for health and other details.