Excellent thread.
Andrew Pierson was elected vice-president at the January 1917 AGM, so it's possible that he would have been elected president in the absence of JFR. Although in the stoush of 1917 he initially sided with Rutherford, he later backed the old board, deciding the president had been wrong in dismissing the majority of the board and appointing his mates to fill the four vacancies, thus stacking it with his sycophantic supporters.
Rutherford's major contributions to the religion were to impose greater central regulation on it, forcing congregations to obey him. He also commissioned The Finished Mystery, which stirred up government opposition and attracted attention to the religion it otherwise wouldn't have received. He sought controversy and was successful at gaining it with a succession of ploys (hate-speech tirades against government and religion, stance against flag salutes, gramophone witnessing, marches, radio broadcasting).
He introduced the requirement to go witnessing door to door and field service record-keeping; introduced worldwide uniformity, created the current concept of Armageddon, "God's vs Satan's organisation", the us-and-them mentality, the picture of a vengeful Jehovah, killed the celebration of birthdays and Christmas. Through The Finished Mystery and other books, he also turned Russell's neat and intriguing system of Bible chronology into a weird set of dates and interpretations that needed constant revision.
Take away all that and Pierson's group would have blended into the background, remaining just another low-key millennialist religion that would have probably died a natural death or remain today as a tiny minority denomination.