And if the humans had lost against the Darkspawn? The Dalish would have been exterminated right along with everyone else. You would have to think very poorly of their intelligence if you believe they couldn't come to that conclusion.
The Emerald Knights could have fought the darkspawn while some went back to evacuate men, women, and children away from the area, perhaps through the Frostback Mountains. There's so much context that's missing from that moment that it's almost impossible to address it with any certainty, and it's more than a little absurd to paint the entirety of the elves of the Dales in a negative light because of it. Loghain's moment at Ostagar proved that things may not necessarily be as simple as we might originally think, after all.
Perhaps more context about the moment might be shed in Inquisition, with the elven Inquisitor in the former elven homeland of the Dales.