As others noted... if Apartheid had had a consistently violent insurgency to wage war against, they could maintain a "state of emergency" stance versus changing the system (and, of course, not wanting to give into terrorism) and maybe they'd have an interest in staging the occasional atrocity to unite their consituents behind the regime.
On a fairly related note...
People have to notice that when South Africa invaded Angola, Cuba intervened, and hey presto, by some coincidence, South Africa was no longer in Angola.
There's a connection there.
Cuban intervention thwarted South African military intervention in Angola, not once, but twice.
Given another twenty years, I'd gladly give odds that South Africa would follow tradition and invade Angola again.
And, Castro, with his rather "internationalist" stance to supporting "the revolution," would have shepherded another large Cuban intervention.
Did the Cubans receive Soviet weapons after they moved to intervene? Yes, and, do note that it was Cuban boots on the ground, late-model Soviet weapons or not.
I find it shameful that the United States was aligned no (must face it) allied with the Apartheid regime in its attempt to stake out a "buffer" zone against the rest of Africa. Americans can claim the motive was anti-communist, sure, but on South Africa's part, it was all about preserving their system of government.
To my knowledge, Cuba is the only country that can claim to have sent military force to fight against the apartheid regime. Maybe this makes Castro's Cuba look very noble, and maybe it just highlights how much a lot of world politics is a filthy nasty dirty vile affair. (As if a couple A-10 Warthogs wouldn't have saved lives in Rwanda, for example, but I digress.)
I agree with an earlier notation that the continuation of the cold war would likely have allowed South Africa more room to wriggle by.
I wonder if Castro would have outlived Apartheid in the above-noted scenario(s)?