More than just answering the question of why Jyn Erso & Co. aren’t featured characters in the original George Lucas movies, it also does a nice job filling in other smaller gaps in the franchise (like providing an explanation as to why Luke Skywalker is leading a group called Rogue Squadron in Empire Strikes Back).
Thankfully Gary Whitta, Gareth Edwards, and the other creative minds behind the project realized that this was not the proper way to bring Rogue One: A Star Wars Story to an end, and the proof that this was the right decision rests in the way in which the prequel movie actually does a proper job enhancing elements of the original trilogy.