I think MSV set the lyrics to tune most of the time. This way he was constrained or could give music only to the sandhams of the favored/limitied lyricists. Fortunately, there were talented lyricists who gave more meaningful words.
In contrast, IR most of the times, gives out the tune first. This way he dictates the sandhams. When ARR came, some discerning listeners appreciated the injection of new sandhams in film music. So this probably is one technical reason for the 'rejuvenation' of the film music industry, apart from more use of the folk or western classical idioms. But I am just speculating. Other, more knowledgeable posters can chime in.