Bordeaux was chosen as the France SC for reasons of playability rather than historical or logistic accuracy. The Modern Diplomacy variant has been around for over 20 years, and has been play tested thousands of times. That doesn't mean that it's perfect, and the Bordeaux situation does hamstring France a little bit, and it's not particularly an accurate representation of France's 'modern' military and economic makeup. With that said to simply swap the SC location from Bordeaux to Brittany has consequences that are not being considered. Giving France a starting position in Brittany without modifying the ocean territories puts England at a terrible disadvantage, and it gives Spain no reason to negotiate over Portugal. You cannot just make these changes in a vacuum without considering the dynamics of what else is going to become imbalanced. It's not like this map was just cooked up three months ago and designed without any consideration to this, or without significant play testing.
The reality is that France is not the weak sister of the Modern board any more than Egypt is on the WW4 map, or for that matter any more than China is in Colonial. Both of those positions are actually anything but weak. France starts the Modern game with four supply centers, in a compact and highly defensible position with excellent alliance opportunities. Just because Bordeaux does not lead to a guaranteed neutral SC pickup does not make it an invaluable position. Modern is a build anywhere map, and the ability to build both fleets and armies at the tip of Iberia is powerful.
Modern's biggest weakness lies not with the odd logistical geographic of France, but with the other side of the map where there is no ability to move fleets north and south. This creates a serious of essentially premade alliances because military progress moving from the Mediterranean Sea to the Baltic Sea or North Atlantic is next to impossible. The solution for this was the insertion of the "wing" unit. Wing units were added to a third version of Modern, which does not appear in the Dip Pouch any longer, but they were widely recognized as making the map much engaging and fun to play.