A problem is that the function determines the candidate ; people who become Presidential candidates are quite egomaniacal already, and are not the kind of people who will once elected decrease their own powers. See how Mitterrand was denouncing the coup d'état permanent in 1965, but was quite happy with being an elected monarch once he got the position. Royal had shown some autocratic tendencies too. See also Jospin who thought he could get elected himself, and thus agreed on the disastrous decision to hold the presidential election in 2002 before the parliamentary elections, and thus forever.

The presidential election is very personal and is not actually fought on a platform. All candidates promise constitutional changes more or less diminishing their powers, but will they actually follow through ?

There is a strong streak in the French electorate for the concept of the powerful single man, since Napoléon ; see De Gaulle, Mitterrand. Parliamentary blocks, coalitions, are quite far from the French political culture, and bad memories of the Fourth Republic are constantly reinforced.

Un roi sans divertissement est un homme plein de misères