I think the main things here it wouldn't be just the dark eldar. If it was a full blown extermination I sincerely believe the Craftworlds would get involved and not just them but the Harlequins as well. Now they are facing 3 "worlds" military.
I actually hadn't considered this. I'm not so sure craftworlders or exodites wold get involved, but the Harlequins certainly would. I suppose it is quite possible that the Harlequins might pressure other sects of the eldar into lending an assist.