A lot of people seem to be overlooking that the Horde leader was already mortally injured and down. I guarantee if the situation were reversed and Varian was the one already mortally injured and near death, along with seeing the situation Genn and others would have called the same retreat. They went there thinking they were taking on a small landing party-sized invasion force, not the bulk of the entire Legion (ex, "That's not an invasion force, that's an army..."). When it became clear that there were 100x more Legion forces there than they anticipated, it went from a long-shot fight to unwinnable and the only options were retreat or annihilation. During the fight Thrall was down as well, and the Alliance barely made it out by the skin of their teeth. Militarily it was a big mistake not to retreat sooner, but it would have been an even larger mistake to fight an unwinnable battle to the end - leaving Azeroth completely open for the taking afterwards.
Think Dunkirk in WW2, the British didn't fight to the end in an unwinnable fight just throwing away lives being stubborn. They retreated and came back to fight another day when the situation is more in their favor. IMO Alliance being upset about the retreat is comparable to saying the troops at Dunkirk should have stayed on the beach and fought to the last man.