I don't agree with "boring theater".
Whatever the outcome of the campaign, flying french planes during the campaign of France can be very interesting and obviously challenging.
For instance, the Breguet 693 family would be a lot of fun to play, quite like flying soviet at the begining of Barbarossa or Japanese in 1945 (and even non jet German in 1945).
The various 1946 mods have french planes (including the excellent Fred D520) and some campaigns which are very nice to fly.
For this, I agree, the only way to have Battle Of France selling significantly would be as an extension to Battle of Britain, that's why having french planes in CloD would have more sense