Other significant differences that mitigate against a Mugabe style "coup" in South Africa has to do with historical roots.  

1. Zimbabwe has only one major urban conurbation; Harare ( population: 1.5m).  
South Africa has Cape Town (3.7m), Port Elizabeth/George (1.5m), Durban/Pinetown (3.4m), Johannesburg (4.4m), Tshwane (2.9m) and Ekurhuleni (3.2m).   
And 4 of these major South African metro areas are NOT ruled by the SACPanc.

2. The Shonas are dominant and make up 70% of the Zimbabwe population.  In South Africa 23% speak Zulu, 16% Xhosa, 14% Afrikaans and 10% English as their "home" language, in total only 63%; this represents a very diverse tribal/cultural landscape, in comparison to Zimbabwe. These tribal/cultural groups are also concentrated along historical lines across 9 provinces (one of which is not ruled by the SACPanc) and the 4 major urban conurbations.