I don't know where you got the idea of unassigned seating for this purported "economy minus" - but surely not even Air France would be mad enough to, as you suggest, send out planes with a mix of pax having assigned seating and pax having unassigned seating.

Air France's systems are designed for assigned seating - it would thus be difficult to to NOT assign seats - and I'm guessing IMPOSSIBLE to assign some seats and not assign others on a single plane.

I have never heard of any airline having a mix of assigned and unassigned seating. If only for the reason that - how do you prevent the unassigned passengers from sitting in the seats that were assigned? Or from sitting in a cabin where they receive a service superior to the one they paid for?

Of course, it could be done by having a new, special "economy minus" cabin at the back, with a big provison on the economy minus pax's boarding passes along the lines of "choose any seat in rows 24-32" or whatever - but I really don't see why AF would concsiously choose such an unworkable strategy.