South Africans are rich and I am sure they are as knowledgeable about bitcoin as you are. These kidnappers are not stupid to ask for Bitcoin worth of $120k. Added to that, they know the value of bitcoin. It is lot easier and safer to leave a note with the btc address than make the victim leave a bag of cash in the street. But still bitcoin is not for these kind of people. Bitcoin is better than that.
How much is known to me SAR is not consider as country where most people live good, there are some rich people like in any country in the world, but their middle class constitutes only 16% of population. If there is few % or very rich people on top of that, we have some 80% people who live in misery and poverty. Parents of that kidnapped boy is not rich and they do knot know anything about BTC.
Kidnappers are actually stupid, they pick wrong family, and they take the boy in front of surveillance cameras - could it be more stupid than that? Yes it can be, they did not got rid of the car used for kidnapping.