Bad timing and bad planning
S.A. never had a chance. They were trying to do to millions of native Africans in the 1960s what we did to thousands of native Americans more than a century earlier.
No way they were going to survive in this world with apartheid. The alternative results are there for all to judge but the country now belongs to the rightful owners.
I can't imagine a scenario that would have worked. They would have had a better chance colonizing Antartica.