The two parties are two separate flavors of ice cream. Everyone loves ice cream, so they argue whether chocolate or vanilla is better. What they really need is an actual balanced diet, but that's not going to happen since most of the voting public is politically a 4 year old child that wants ice cream...
They can vote strawberry, but there is no chance of winning. Many will still do it anyway because it is a sort of symbolism or alternative to invalidating the ballot, or because they want to show that a significant number of strawberries are there in order to encourage people to vote strawberry in future elections. But many others will, in the end, vote for whichever majority flavor they would much rather see winning. If they are serious about getting more strawberry, they may do things like trying to win the vanilla primary, or positioning themselves as providing vanilla votes in exchange for a strawberry swirl.
And the two-party system continues.
Meanwhile, a tiny number of people believe we should all starve, or eat only gravel, or human flesh. While different, these are not necessarily more "balanced" suggestions. Some would like to bypass democracy to install their own like-minded buddies, which isn't very balanced either.