In complement to Von Kar’s answer : I think you’re misunderstanding the meaning of this sentence. Here, « a été long » means the prosecutor spent a long time talking about it. Like when you say “I won’t be long”, you mean that you won’t take much time to do whatever you have to do, not that you will lose a few centimeters (except perhaps in some very specific contexts). As far as meaning is concerned, it’s not a state or a change of state, it’s not a continuous action in parallel to the sequence, it’s a ponctual action in a sequence. You could replace it by « il a passé beaucoup plus de temps » or « il a développé plus longuement ».

Keeping that in mind, « était » instead of « a été » won’t work. « Fut » would fit the meaning and fill the same role as « a été » in the sentence, but in this instance it would break the sequence of tenses. Here, the tense of narration is the compound past, you can’t have a simple past coming out of nowhere.