Having thought about this a little bit... I actually can't see this affecting the political landscape much.
Here's my take on this...
1) Mmusi Maimane and Herman Mashaba form a new party and it draws some of the black voters away from the ANC, the DA and maybe even the EFF. The DA will lose a few percentage points, but will drop back to 2014 levels (which really isn't that much different).
2) Mmusi Maimane and Herman Mashaba join other predominantly smaller parties like the EFF or COPE. Same result as above.
3) Mmusi Maimane and Herman Mashaba fade into obscurity....
So really... I don't see this thing changing the political landscape much. If anything DA will lose a few percentage points, but the ones who stand to lose the most are actually the ANC....