It appears South Africa's birth rate is falling. According to the SAIRR, the birth rate will drop 17% between 2010 and 2040. Which they seem to think is a bad thing. Mostly because they fear that a birth rate below 'replacement level' will shrink the workforce, as well as increasing the dependency ratio because there will be a larger population over 65 (12% instead of 5%). Apparently the SAIRR researchers live in Japan. South Africa has a massive youth unemployment problem, the economy simply cannot absorb a good portion of the labour available to it and the current dependency burden - largely because 9 million children require state support - is probably far greater than the slightly increased old-age burden would be (as the article points out).
On top of that, the argument that the workforce will shrink and that is bad is a ridiculous one. Southern Africa, as a whole, has plenty of people who are ready and willing to move to South Africa and do the jobs, on the rather far-fetched assumption that the South African labour force shrinks beyond what is sustainable. We know this because they're ALREADY moving to South Africa, in spite of the country's huge unemployment problem. And because of the uneven rates of development in Southern Africa, it's highly unlikely that their countries would have reached that magic level of middle-classness where they reduce family size by the time South Africa moves from xenophobia to needing more people to employee. Why on earth would it be fundamentally necessary for South Africans to replace themselves with other South Africans? The only way there could ever be a labour shortage would be if the borders were completely closed to all other workers. This seems unlikely. It would also be silly.
Luckily for us, the (glacially slow) movement of SADC is in the direction of allowing more labour movement, not less, so even in the unlikely event that South Africa's own population shrank below the level required to run the economy, there is a pretty good chance the country would still be able to make it.
H/T the awesome @sarahemilywild