Ah yes, the African model of one party democracy. It sure works great in Zimbabwe!
One party democracy is hardly unique to Africa. One need only look at the dominance of the PRI in Mexican politics and of the LDP in Japanese politics for many decades to see models similar to that of the ANC in current South African politics. We've gone thru periods of one party democracy in the United States such as after the war of 1812 when the only viable party was the Democratic-Republican and during the Jim Crow era in the South when if you wanted to be elected, you had to be a Democrat.
The National Party never had a realistic hope of regaining power, as it or any successor would always be tainted by the memory of apartheid. It is hard to say how long it will be before South Africa returns to multi-party demoracy, but it will eventually, and there is every reason to expect that it will be a peaceful transition such as happened in Mexico or Japan. I'm not particularly worried about the prospect of South Africa going the way of Zimbabwe. I won't say that it is impossible, but it is extremely unlikely.
