It was a pretty surprising reveal to say the least. I think many Dalish would be completely in denial about the whole thing. They built up their entire group identity around being the only remaining "real" elves, so finding out they're descendants of Arlathan's slaves will probably result in a profound identity crisis for some.
On the other hand, I think all in all it doesn't really matter that much. The past is the past, and even though they may have gotten their far-away history wrong, it doesn't change the fact that the Dalish went their own way and carved out their own place in the world. Besides, aren't they more about continuing the traditions of the Dales, i.e. their country after Arlathan? (Not sure about that, so feel free to correct me if I'm wrong)
In a very weird way, I also think it's good Arlathan more or less imploded. It casts a whole new interesting light on Tevinter. Basically they've now been reduced to upjumped scavengers 
Edit: One of my pet peeves with this part of the story: Sera's reaction to this whole reveal when in a romance with a Dalish inquisitor could've been written a whole lot better. You have to take an extreme stance on the subject no matter what. Either you're a raging elven supremacist, or you have to say "screw my whole life before now, I wanna be human". Slightly off topic, but I just played through that part and I wanted to share my disappointment. 