I recall the period of the release of Graceland. At the time world opinion was building against the apartheid policies of the South African government, at least among the populace of the West. Due to its rich deposits of strategic minerals, some of which (such as chromium) were only available in usable quantities from the Soviet Union, the governments of the First World were unwilling to pay more than lip service to their expressed disapproval. In this context, suddenly an American musical celebrity arrives in South Africa. It would be easy to understand the response of the African National Congress and the young man in the street that Simon had broken the cultural embargo, but it can be argued that this was one more action that brought attention to the plight of black South Africans. The popularity of the album, whether or not it actually accurately depicted the music of the land, focused more attention on these artists and the people they represented. Each such action chiselled away a bit more of the wall that hid the actions of the apartheid supporters in trying to maintain their hold on government. By the early 90s, this wall was nearly down, and as an example of this, the imprisoned Nelson Mandela was released after twenty-seven years in confinement. A few years later and the government changed, leading to majority rule, and the ending of apartheid.
I refuse to suggest that Graceland was solely responsible for the ending of apartheid. Many things contributed, from the Olympic ban from the 1970s, to the cultural embargo in the 1980s, to Graceland's release, to the world's acceptance of Ladysmith Black Mambazo, to the Regents of the University of California being pressured to divest the University's endowments of South African shares, to the ongoing actions of the African National Congress in revolt against the government's policies; each was necessary in their own ways to increase the pressure to intolerable levels. Finally, de Klerk acknowledged the legitimacy of the ANC, and the floodgates burst.
Should Simon have gone? At the time, his career appeared to be in decline; he needed an infusion of energy and some means of stirring his creative juices to result in a form that would excite his audience. In addition, in his early years, he and his partner, Art Garfunkle, produced music that often reflected the politics of the period. In this context, going to South Africa flew in the face of the majority of his peers, while initially appearing to support the downtrodden African musicians. I don't think that Simon could have made any other choice, once he listened to the cassette of South African music. It's never clear what the final effect of a given action might be, because so many factors modify the end result. With the pressures of the world's attention, the apartheid supporters fell, and South Africa was born anew.