After a week of controversy over the prospect, we are now told that Robert J. Brown won't be named ambassador to South Africa after all. No matter. From the start, it was the wrong controversy over the wrong man.

If the Reagan administration is serious about changing its South Africa policy, then the man begging to be replaced is not the retiring Ambassador Herman Nickel, but Chester Crocker, the assistant secretary of state who dreamed up the policy of "constructive engagement," which the administration is so reluctant to abandon.

Nothing is clearer than that Crocker's big-carrot/tiny-stick policy has been a double-edged failure. It has failed in its primary objective of giving the South African government time to settle such pressing controversies as Angola, Namibia (remember Namibia?) and, of course, the apartheid system, which makes second- and third-class citizens of the 85 percent of the people in South Africa who are not white. And it has painted the U.S. government as such an enemy of the aspirations of the nonblack majority that even such staunch antiapartheid officials as Del. Walter Fauntroy and Rep. Bill Gray could not entirely escape black hostility during a recent visit.

What would the dispatch of Bob Brown, a conservative black businessman inexperienced in diplomacy, have done to improve the American role in avoiding chaos in South Africa? Nothing. And it might have done a good deal of harm.

Given the improbability of sweeping change in official U.S. policy, South African blacks would likely have viewed Brown as no improvement on Nickel and perhaps, precisely because he is black, worse. At the same time, state President P. W. Botha might have interpreted Reagan's appointment of a black ambassador as a gratuitous insult from a man he thought was his friend.

If Reagan wants to signal major change in the policy that has worked so poorly for U.S. interests, let him replace the man who created that policy.

Economic sanctions and disinvestment, the other piece of the current controversy, are other wrong issues, not because of logical doubts as to their efficacy but because, logic aside, they are inevitable.

Secretary of State George Shultz makes a valid point when he insists on the importance of maintaining diplomatic relations with South Africa lest we lose the ability to exert any influence there. But nothing in Rep. Ronald Dellums sweeping sanctions bill, which already has passed the House, calls for severing of diplomatic relations. It calls for complete divestment, an embargo on all trade and the severing of economic ties with South Africa.

You can argue whether it might have been better to enact phased sanctions, tying each tightening of the economic screws to South Africa's continued intransigence. But the argument has all but lost its point.

Economic sanctions and disinvestment are already under way. Look at the number of American institutions that have been pressured into purging their portfolios of stock in companies doing business in South Africa. Look at South Africa's devastated economy, its plummeting currency, the result not of official sanctions but of the loss of investor confidence.

Philosophical considerations aside, new foreign investment in South Africa is simply bad business. And the investments already made will come under increasing pressure -- unless the government there moves quickly to dismantle apartheid.

Unfortunately, Botha, caught between South Africa's political moderates (including its business community), who are calling for major change, and the hard-line Afrikaners, who are calling on him to stiffen his resistance, may find it impossible to do what has to be done in order to avoid bloody chaos.

Reagan could save him from his dilemma. If Reagan can communicate to Botha that the combination of American political pressure and South African intransigence has made it impossible for the United States to continue its supportive role; that any further assistance, diplomatic or otherwise, must be predicated on demonstrable progress toward ending apartheid; that the minimum requirement for such progress is the release of key political prisoners, unbanning of the African National Congress and the institution of serious negotiations -- if Reagan communicates these things, then it could give Botha the political wherewithal to do what must be done.

It is no longer possible for the Reagan administration to save South Africa from its impending disaster. But a tougher U.S. policy, unequivocally laid out, might provide the pressure for South Africa to what is necessary to save itself.