I think the ENY factor is the easiest way to implement a plane availability factor, no need to alter the game with a bunch of long drawn out coding. Here is how it works (and used to work, iirc):
For each fighter hanger destoryed the ENY of that specific base is raised by 10 points (or other such factor, but make it legit). This would simulate an airfield having less to choose from for available defending aircraft and in many cases the "best" were often in the fewest numbers due to being used elsewhere. If there are 4 fighter hangers and 3 of them are destroyed, then we're looking at a 30 ENY score (wich needs to be adjusted to allow THAT score and higher, and not THAT score and below if increments of 10 are used). If this system were used it would allow the game to get away from the "all or none" system of play when it comes to OBJ and base strat targets. If the major strat targets can have a sliding scale of damage... I would think that the same could be applied to the hangers (and base strats too).
On the same note... I think the base strat targets should be a sliding scale as well (destory 1 fuel bunker = no DT, 2 fuel bunkers = %75 fuel, etc). This "all or none" is becoming too "same-same", and introducing a sliding damage scale for the base strats and hangers would limit the number of 1k bombs, DT, lancs-stuks, and "uber" fighters we see without messing with the ENY and OBJ scores HTC seems to be so hesitant to adjust.