And then Horde respawn 20 at a time dozens of yards away and Alliance respawn 10 at a time hundreds of yards away and thus can never hold it long enough to capture it.
No matter what, this is always going to come back to the fact that Alliance cannot capture IBGY because of this ridiculous numbers advantage. Even if Blizzard closed the back door to the bridge, Alliance still cannot win because they still cannot capture IBGY. They can just hold off the Horde for a while until everyone gets bored and leaves.