on the issue of evacuate or not, about 200m behind where the B747 came to a stop, is a satellite Fire station at JNB, and these crew would have heard the sound of the building being ripped apart, and would have been on scene within a short space of time.
so short a space of time, that it would have been equivalent to the time taken by the flight deck crew to evaluate (with liaison from Cabin crew due to not being able to see the extent of the damage), and by the time the flight deck had evaluated the situation, the first fire crew would have been on scene already to assist in the decision making model.
that being said, if it was assessed as catastrophic or imminent outbreak of fire, then an evacuation would have been sensible and prudent, as the main fire base is in the centre-field, and would have taken a while longer to get on scene.
but as it turns out, apart from 4 ground staff injured, hurt pride, material damages, all turned out OK in the end.