You sound like a reasonable person, thus my post was not addressed to you.
Unlike greymane, or jaina, or so many alliance players, I am NOT blaming the alliance for what happened. Both sides weren't properly prepared to fight the legion, the horde, the alliance, and the argent crusade had a collective moment of stupidity or maybe just overinflated ego, as they drastically underestimated what they were going up against. And neither side could have predicted the spaceships. Those things are new, first time legion has ever used them, and they were the primary reason for why the offensive failed. There's only so much you can do when you're being bombarded from the sky.
I can understand why the alliance sounded the retreat, the main reason why I pointed out the circumstances of said retreat was as a counterpoint to rob that the horde was "pathetic" for retreating.
I retorted with a "At least we only retreated when we were almost dead. The alliance was still in fighting shape, but when they saw shit going sideways they retreated immediately".
I can understand why they did it, just like most of the in-game alliance (minus Jaina, greymane, rogers, and some players) understands why the horde withdrew. That doesn't mean I don't think its kinda lame that they didn't give it a little more oomph in their effort.
The plan was "horde buys time for the alliance to charge at gul'dan and kill him", the horde held their end of the bargain, we held the tide of demons on that cliff, but the alliance just stood there. They didn't charge in, they waited for gul'dan to summon more demons and just stood there drooling, only killing the demons that came close, with varian delivering cheesy speeches instead of charging in. Sylvanas had to scold varian to shut the fuck up and kill gul'dan already.
Though to be fair to the alliance, this was a doomed enterprise from the beginning. As we saw from the audiobook "The Tomb of Sargeras", Gul'dan opened the portal long before we ever got there. Even if the alliance had managed to kill gul'dan, that portal wasn't magically gonna seal itself up. Gul'dan wasn't summoning that COLOSSAL tidewave of demons on the cliff, those were coming up from the legion itself.
had gul'dan been slain, it would have dealt a big blow to the legion, but wouldn't have halted the invasion, the horde would have been forced to retreat, the demons would have overrun the alliance anyways, and they would have been forced to retreat as well. Its not like killing gul'dan would have magically ended the invasion.