The real reason Zuma cannot be expelled by the ANC is that any sanction pressed against Zuma and the Zulu-Nostra by the ANC, will cause deep humiliation in the Zulu nation and result in them withdrawing support.
The ANC would simply lose a massive chunk of their support base and definitely lose the 2019 elections. Zuma knows this, Gwede and the NEC knows this, and therein lies Zuma’s power, until such time as he is removed in a dignified manner.
They mutter about this, but cannot do anything to save the ANC - the proverbial rock and hard place applies. it is so contentious that it is 'unmentionable' and Gwede can only pretend otherwise.
We forget that amongst many South Africans, tribalism is at a constant yet controlled simmer.
It is only the focusing on black/white divisions and disparities that diverts attention from the black on black political violence as experienced in the lead up to 1994 elections.