Large reserves and production capacity means market control, i. e. they can control, or at least set an upper limit, on price. This allows them, among other thi

Large reserves and production capacity means market control, i. e. they can control, or at least set an upper limit, on price. This allows them, among other things, to scare potential competitors away from trying to compete. It also strengthens their position in OPEC. And, IIRC, there may even be some provisions in international agreements that tie quotas to reserves. Counterintuitively, a high price may actually be scarier for SA than a low price. It may cause competitors to invest in exploration. And where fixed, initial cost make up a large part of costs, some competitors may continue to produce even if prices fall, and even if the investment was based on wrong estimates and may never actually reach break-even. SA running out of oil would also hasten efforts to replace oil with renewables.