Robert was not a usurper, nor was Hugh Capet. Napoleon is a different case. Robert did not usurp the throne, he was chosen because his relation to the Targaryen dynasty was the closest one of those who held the rebellion against a king who had betrayed his vassals. Hugh Capet was elected by those who had the power to do so and members of his family had been brought to that position before. Robert's kingdom did not collapse, it still stands in spite of a couple of wars and will remain in Cersei's grasp after the fool in the North and the lizard bitch get what they deserve.