Materially, the South Africans could have kept it up. Politically, there was no will to continue. You are correct that the problem was essentially one of consumer economics. White South Africans were not about to compromise further on their standards of living.The apartheid regime could have not lasted beyond the 90's because of its high debt which the current regime still had to pay at the expense of having to put more resources into improving the standard of living of the non-white members of SA.
By the mid 80's, the black population had also become militant to the point that the white population realised that if the country had to be brought back from the brink of a civil war, apartheid had to be abolished.
I think, however, that the state of the economy is contingent upon the prior collapse of the other settler regimes.