Junk status does not prevent anyone investing in a country. For certain classes of investment they're required to sell if the status is junk (which itself has many levels), but otherwise they can still buy equities, invest in real estate and so on. The investment just becomes higher risk.
You're forgetting interest rates. I can't comment on the purchasing power in the 70's but from what I understand was during the pre 94 era the interest rate was significantly high relative to what we have now. So based on basic economic theory, aside from formal fund managers constrained by rules such as credit ratings, anyone else could drop money into the rand and get high returns, which leads to high demand for rand currency and pushes up its value relative to the US dollar. Bring back those interest rates and tomorrow you should see a much higher demand for rand currency (in the short term anyway) at the expense of the local population imploding in the sudden debt trap.
Edit: So SA had good purchasing power but no one to buy from due to sanctions.
I don't think it was that great in the 70s in South Africa. The US Dollar exchange rate might have been better, but import duties tended to be extremely high and the middlemen who imported products put massive markups on them. Anyone who complains about some product not being available in South Africa today has no idea of how restricted choices were before the 90s. But the 70s were still pretty good compared to the 80s which is when things really went to hell in South Africa.
I think people like to overlook all that was bad in South Africa before the 90s. And for some reason they're incapable of acknowledging that after the fall of Apartheid things actually became better before they started to go bad. Because honestly in many ways South Africa became a far better place during the 90s and carried on being so into the early 2000s. It really only started to go the other way at some point after 2005.
I'm just stating the inevitable dark cloud that's looming. Most africans are happy with how the country is being run and they won't mind junk status as long as they can still enjoy their inflated salaries for being previously disadvantaged.
I had not realised all those protests were expressions of happiness.