20% A ... 20% B ... 15% C ... 15% D C ... 15% E C ... 15% F C ...
"But then they can't vote for their prefered candidate which was the whole point"
Well, some people can. D, E, F could still get a few percentage points. More importantly, I don't think we would see convergence to a 2-party system.
Unless I am missing something, it looks like at least 3 parties could be sustained.