1) I thought ENY was to help prevent hording by one side over the other. Tying it to fields owned wouldn't fix that at all.
2) Tying it to fields owned would be inclined to stagnate the arena and slow the resets, as the closer to reset a country gets the worse planes they can fly to finish the job. Forcing people to fly the same maps for longer periods of time probably isn't in the best interest of the game overall.
3) In my experience, the moment ENY goes above 5.0 the percentage of people sitting the tower begins to build like mad - half the reason half the people in the screenshots are in the tower is they are waiting for a chance to grab their ride again.
4) If you based it solely upon numbers in flight, large groups would game it by simul-launching.
5) The good thing about ENY is that ENY works - at least to some extent. (The bad is that there is lots of situations where it doesn't help, and even hinders).
6) It would work much better though if it were localized, limited based upon what's in the immediate sectors around your airfield/vehicle base and not upon the overall country players. It's very frustrating to be ENY constrained while being locally outnumbered, because most people on your country are trying to overwhelm a field on the other side of the map with sheer numbers.
In fact, in this situation, ENY reinforces the horde mentality. Your choices are to change sides and fly with the other horde, change fronts and fly with your own horde, or continue to get hammered by the overwhelming odds until you get tired and log.