As I've said before, because of the need for gameplay balance you mentioned the more the Horde is beat down in the Siege of Orgrimmar, the more story focus they'll get over the next expansion or two building themselves back up. Somehow I don't believe this is what Alliance players really want.
Best for the Horde to come out of the Siege as intact as could be reasonably expected, with a well-respected and independent leader at the helm. Now the loss of the Kor'kron is non-trivial and could and should have implications going forward. But I think those implications can be dealt with without unbalancing the narrative.