I would disagree. The regime, on its own, lifted the ban on the ANC, freed Mandella, ended Apartheid and cleared the way for a new constitution giving equal franchise to all South Africans under the leadership of F.W. de Klerk.
Having lived through the entire era, about 30 years, I would suggest that the perceived inferiority of blacks in the early sixties and the desire to ensure domestic stability led to the introduction of Apartheid. With the civil rights debate unfolding in America combined with the globalization of human rights and consistent opposition by the populace, the goal of stability was never realized. Never once have I read that South Africa was suffering from the boycotts economically.
You might find this 2005 speech byF.W. de Klerk very interesting in this regard.