Floor-crossing was done with the co-operation of all the major parties at the time.
None of the other options required a two-thirds majority.
Changing their own political head (although it effects the rest of the country) does not require the rest of the country's approval. Similar how the DA changed it's head from Tony Leon to Helen Zille - did they have to ask the other parties whether it was okay by them?
The dissolution of the Scorpions required a simple majority, rather than a two-thirds majority.
Your examples are flawed. There are other things that they've done that did require a two-thirds majority, such as redrawing the borders around Khutsong.
And just to be clear again: I will not be voting for the ANC, and I think that two-thirds is too much power for any party. I just think we should drop the scare-mongering, and keep our arguments factual.