Hermann Giliomee, along with fellow revolutionologist Theda Skocpol, notes that South Africa does not and is not about to fall because the government remains domestically strong, stable and autonomous and it will continue to do so "if the police and army remain coherent and effective in controlling the population, and if the state continues to collect taxes and attract police and military recruits."
Certainly these factors are necessary for any government, however blatantly repressive, repugnant and illegitimate, to remain viable and in power, but also much more has to be present in order for it to survive. The Shah of Iran had considerable oil revenues, a large pool of recruits, a strong, loyal police and army as well as a staunch U.S. ally to back him up, but his regime rapidly and surprisingly disintegrated within a matter of months when confronted by prolonged, massive street demonstrations against it. What happened dramatically showed how quickly a revolution, once started, can engulf an entire society.
It's true there are critical differences between Iran and South Africa in that the latter has a strong, affluent, determined white minority, a hard, tough core that supports the government nearly 100%, while the Shah not only had no such constituency outside his own created state and army, but was seen as having sold out to the West and of being its puppet, a view that wasn't altogether false.