Ultimatly all regimes that do not command majority respect and over reach themselves militarilly usually collapes as their finances collapse. Up to the 90ies SA did not command majority respect and was was involved in several wars aound it that (Namibia, Angola, Mozambique) it would never win. It ois probable the collapse of the Soviet Union allowed a suitable hiatus for all concerned to try some thing new
If the whites did not vote YES in the 1992 referendum where the question was asked whether the government should continue with negotiations with the ANC for multi-racial elections then white rule would have continued.
Militarily they had more than enough to deal with any issues locally and had already concluded the war in Angola and withdrawn from Namibia. There was no more financial burden of external wars.
Externally, no foreign powers would have interfered due to having atomic bombs.
The townships and locations were and still are designed to be cut off with minimal manpower and carpet bombed or shelled. Due to the close proximity of housing minimal fires would be needed to have caused massive destruction.
Whites on the other hand were and are less concentrated.
Don't forget that a lot of the police were black and that blacks were also used in the special forces and elite fighting groups such as 32 battalion.
Under PW Botha, the plan was for an extended Federal structure with blacks represented in the South African parliament as well, albeit separately.
And with the Homelands, the blacks had their independent states and self-rule, but unfortunately not recognised by most of the world even though they had their own parliaments, uniforms, military, police, passports, bureacracies etc.
And contrary to popular belief they held the most fertile lands as their ancestors knew this, most of SA is desert or arid so the figures representing the percentage of white South Africa were misleading as most of it was uninhabitable or uninhabited while the Homelands had good land.
Also, tax from South Africa went to develop these Homelands.
North Korea is still going, and I see no reason why South Africa under white rule would not have carried on.
Anyway, racial classification ended in 1991 and most apartheid laws were gone in the 80's except for living in residential areas and their were unclassified zones were anyone could live as well.
Before 1991 I was at school with blacks and coloureds as well and in the 80's had Taiwanese people living in my neigbourhood. Many of the universities in the 80's were also multi-racial and Mandela and the rest of the terrorists all got their degrees in jail or house arrest under government payment.
