Nothing against SA sides joining the Pro12 in principle - it makes more sense than North American sides as they'd be better, and in the same timezone at least - but not like this. It would be immensely damaging to the Pro12 as a brand to be seen as a dumping ground for teams "not good enough" to be in Super Rugby. Whether or not they'd be competitive in the Pro12 (I think they would) is besides the point; we don't want to be branded as a Second Dvision and that is what we would literally be if the South Africans split their teams between the Pro12 and something else. The best players would inevitably gravitate to the Super Rugby sides.
It should be all six SA sides (or as many Pro sides as they want to keep), or none of them.