Rolex was not the biggest till 1974. Omega out numbered Rolex cosc certified chronomter from 600000 units as far as I can remember. Rolex as I said before always prefered to have something sure over something fashionable and not so reliable. This is where comes from Rolex dominance, to have a conservative design and a proven and very reliable movement made to last generations. Also Rolex was the first brand to test its watch to cosc before called bureaux de contrôle suisse des chronomètres and especially at kew teddington observatory where its watch won the first clas A certificate on 1908. Later Rolex again revolutionned the world with the screw down crown and screwn case back and the first waterproof wristwatch. Mercedes Gleitze swam across the channel with a Rolex in 1924 and it was working perfectly. In 1930 again an innovation, the first rotor automatic unidirectional winding movement built by Rolex. In 1953, the first diving watch with screwn down crown, radium hands and indexes and waterproof to 100m, the sub. In 1970 the first watch with a completely automatic helium escape valve waterproof to 610m till 1978, and then water proof to 1220m, the sea dweller. Of course one of the most important revolution of Rolex was the microstella balance introduced in 1957 and till today the reference of Rolex superiority.
Hope this answers your question.