Why not B?
Because every city would now be an open target to air units, resulting in the meta shifting towards a DS/SM/IMP/Blitz spamfest. At higher levels of play most semi-pros and professionals wallfuck and meticulously plan their moves, meaning most walls are ineffective anyways. You would be crippling walls with an update like this.
If anything, we should bring back Airwalls in the form of the Fighter Air: Main Defense unit, and piggybacking off of Witch Doctor's cycles/weaknesses idea, said walls would protect against other air units and ground units, but not naval ones, meaning that a solution exists to this problem if you are the attacking player, giving a respectable new layer to gameplay. I just want flowerpower back man.