This is the part that I've always found amazing (not that I dispute that it's true). Did white South Africans not realise that in the 1970's and 1980's that they (along with Rhodesia until 1980) were probably the most hated nation in the world, even in the West? You would have thought that this would surely have had more effect on white South African public opinion than it did.
As for my opinion on the ATL, I think the continued existence of the USSR would set likely mean apartheid would continue for longer, but only a few years longer. I think apartheid would definitely be gone by 2000.
As the son of a Rhodesian and having studied Africa since my teens, I can say one thing about many white Rhodesians, Afrikaners and "uitlander" South Africans - they really don't give a flying f**k what anyone else thinks about them. They all knew that apartheid was unworkable, but the problem was the alternative - the black African states headed into the shitter real fast, particularly in the Congo and Angola where independence kicked off a civil war. The petty apartheid laws started being dismantled in the Cape by 1975 and Johannesburg was comfortably integrated (though it was technically illegal at the time) by 1980. It was as black as Nairobi or Lagos by 1990. The areas which stayed most resistant to change were the rural areas, where in many cases the segregated schools didn't end until as late as 1997-98.
If apartheid had managed to survive until 2000 and see what Mugabe did in Zimbabwe, there would have been a large portion of the nation which saw that and all of a sudden wasn't too unkind toward apartheid - not just whites, but also the Indians, Asians and coloreds.
In OTL, the negotiations were going slowly and fitfully through the chaos and violence on 1991 and 1992, but when Chris Hani was gunned down in Johannesburg, the country went right to the edge of civil war, and both the ANC and the NP realized that if they didn't sort this out, all hell would break loose. By then, most of the NP had accepted that apartheid was gone, and even if they had to reimpose it they wouldn't be able to - Natal and much of the Transvaal would be an epic mess within days.
The USSR surviving might have kept the communist boogeyman alive, but the facts were that in 1990 the USSR wasn't exactly providing a heckuva lot of support to the ANC. The SACP was a major player in the ANC, though Mandela and most of the moderates of the ANC knew full well that Pretoria would never accept a government that was communist or anywhere near it. The ANC had enough trouble getting Pretoria to accept Joe Slovo and Chris Hani as part of the negotiating team.