The Fifth Republic system is very ad hoc and the kinks have had to be worked out by actually trying things. Can the Republic, a Gaullist power vehicle, survive without its founder? Let's get rid of Charles de Gaulle and find out. Can the Republic survive a non-Gaullist President? Let's see what Valery Giscard d'Estaing does with the government. Can the strong presidential system survive with a President opposed to its very existence? Let's see what Mitterand gets up to. What happens with a Socialist President and a Gaullist PM? Let's have Mitterand and Chirac figure things out. What happens when a fascist makes it into the top two presidential runoff? Let's hope the electorate isn't terrible and let the voters decide.
Are you saying that the role of the president and the role of the prime minister had been changing since 1959?
I think there's a case for that, yes, though to a lesser extent today as the PS has slowly gotten over its distaste with the Fifth Republic as a concept, and not even entirely there. As recently as 2007 Segolene Royal floated the idea of a Sixth Republic during her doomed Presidential bid.
