No Border War might prolong Apartheid, but it isn't going to last forever. It is, after all, an inherently contradictory system: disenfranchising huge amounts of the populace is still having huge aftereffects on the South African economy today. And, honestly, once the noose started to tighten around the Apartheid regime with regards to major, across-the-board sanctions in the 80's, the Apartheid system's borrowed time got even shorter.
In short, I see things like the economic strains created by the Border War as a symptom rather than a cause of the disease: Apartheid as a whole was untenable. And, really, the longer it goes on, the more likely it will be that people like Mandela get marginalized and the whole thing ends violently.
This is partly true. The other half of this is that the ANC and members of the apartheid state's senior members and its business community began talking in the early 1980s. The 1983 constitutional reforms in South Africa were an attempt to forestall the inevitable, they knew that problems were coming even then, and the decision to let go of Namibia began to be made in this time period.
Extending apartheid couldn't be done much beyond OTL even if you butterfly the war. The debt problems didn't hurt South Africa much until the sanctions began to cripple its trade and cause economic problems for the whites, which the border war, for all its costs, wouldn't change very much. If you want to do that, you need to get the colored and Indian populations on the side of the apartheid state and outright annex Namibia, but the problem there is that these populations combined with the English whites could easily break the National Party's hold on power in South Africa, which is why they'd never consider these ideas.