Let's look at the argument for a fair rotation. First, the parameters and assumptions.
Let's say for simplicity sake the FSO takes 200 peeps.
The scenario has a 60/40 split in favor of allies (120 allied / 80 axis)
There's 5 large squads of 20, 6 medium squads of 10, and 8 small squads of 5.
3 large squads, 4 medium squads, and 6 small squads want to be allied (130 people)
2 large squad, 2 medium squads, and 2 small squads want to be axis (70 people)
In this scenario, we need 10 people to take the side they don't want. For the first frame, 2 small squads were assigned to axis. For the second frame, a medium squad was assigned axis. And for the third frame, 2 small squads were assigned axis. If we assume the next month is similar except that the medium squads go to the side they don't want twice and 2 small squad do it once, then we end up with 4 medium squads being assigned to sides they don't want every 6th or 7th frame, and 6 of the small squads have to change to a side they don't like every 4th or 5th frame. Meanwhile, 2 each large, medium and small squads get to stay axis. If I understand correctly, this is what is not fair. Fair would be every squad changing sides (in this scenario) every other or, at best, every 3rd frame. So, in a Pacific scenario when JG11 or Kommando Nowotny is willing to fly Japanese so more allied oriented players can fly their big blue birds, fair would be to force the allied squads to fly Japanese. And in a European scenario, it's better to force everybody to spend at least half the time "out of their comfort zone" instead of the obvious solution of minimizing the number of people who are forced to change sides. Remember, to make this fit your definition of fair, every squad will be changing sides at a minimum every other or every third frame. Not month, but frame. I'm guessing you wouldn't be any happier with that setup.
I don't know what might be a solution to your discontentment, but I doubt what you are implying as a "fair" rotation would satisfy you in the long term.
Regards,
Hammer