I'm not an expert, but I believe this is a misunderstanding of Marxism. According to Marx, the modern State is itself dictatorial ("the existence of the state is inseparable from the existence of slavery" -- Critical Notes on "The King of Prussia"), with the Bourgeoisie being the dominant class.

The dictatorship of the proletariat is a transitional revolutionary period in which the State still exists (hence the dictatorship), but the proletariat becomes the dominant class instead. This is followed by the abolishment of classes and of the State itself.

(Note: I'm not actually interested in discussing the merits of Marxism, I just think one should properly represent a position before having an opinion on it)