Sure, wiping the Horde out isn't realistic, but either is walking away with nothing to show for it. Considering the Alliance could have wiped the whole Horde leadership out right then and there, it's pathetic to just walk away. There's nothing to show for it, just a threat and a Horde left to strengthen itself. A forced surrender of Gilneas would have been the least they could have done.
Instead, they stuck a big middle finger up to the Alliance players and told them to spin on it.