That's not really true and if there's one thing the ANC has done exceptionally well, is to frame itself as the only opposition to apartheid. By the 1970s, the ANC's internal resistance to apartheid was crushed. The vacuum was filled by organisations like the Black Consciousness Movement and people like Mangosuthu Buthelezi and Inkatha in KwaZulu-Natal, and even the white parliamentary liberal opposition. By the 1980s internal resistance had coalesced around the United Democratic Front (UDF) which had evolved without any assistance from the ANC.
The ANC did a fantastic job of doing a reverse takeover of the UDF once it was unbanned, and, at the same time, through propaganda and violence, setting itself up as the only legitimate opposition to apartheid.
The ANC emerging as this hegemonic opponent to aparheid was not inevitable, not by a longshot.
You're right, I should have more explicitly clarified that as "past a certain point" there was no replacing the ANC but thank you for sharing all of that, I legitimately didn't know about some of the organisations that you mentioned.
Though, regarding the Inkatha Freedom Party, my assessment of them could be entirely erroneous, but it seemed to me like the IFP always had a bit too much baggage to really be able to emerge as a national South African movement. Primarily because Buthelezi owed his position to the Apartheid regime and in many ways owed his entire power base to the existence of the bantustans and by extension Apartheid as a whole.
That said, I find the IFP a really fascinating part of South African history and wish I knew more about them.