If the National Party hadn't won the election in 1948 and then promptly proceeded to seize the idiot ball, then a white minority government that gradually expands the franchise but still has whites dominate the country might have been hold things together until the 2000s. And such a one might stay in power by aligning themselves with the PRC for trade (and India if they have the brains to expand the franchise to Indians) and Russia to buy weapons from, making sanctions toothless.
Apartheid though was doomed from the start.