Winging it presents one distinct disadvantage one could well run into, ie the lack of lodging. In France you could still lower the odds of that happening in choosing your itinerary, even in the summer, which is the time I'm there.
As far as I know Brittany offers a good chance for "winging it." as do Northern France and Lorraine, given the image of Brittany in the summer. When you walk around and explore towns in Northern France , say in Amiens, Noyon, Arras, Maubeuge, Soissons, etc) , observe who is there in the train station, the centre-ville...any obvious foreign tourists ? If any, they are French. That was my experience this summer. Only in Rouen and Troyes did I see Anglophones in any number.
No, this desire to "wing it" is not silly, If it is, so what? Do it anyway. In a way nostalgic and may not be practical, as regards to expenses. If you do decide on this option of "winging it," I would suggest that in each town/city you check out the Tourist Office. I found it very useful in talking to the friendly and helpful staff in every T.O. I entered, be it in Rouen, Albert/Somme, Troyes, Thionville, etc.
Would I do this option? Not normally, but I am a lot more tempted to do so in France (and most likely will do some of that next summer when I get back to France) than in other countries. "Winging it" is a minor option.