I understand there may be some opposition on the Horde side (as some Horde players want to win, even though I think losing hard before coming back with vengeance would be a more powerful storyline), but why would any Alliance players, no matter their stance on the faction war, oppose the extermination of Horde races by the Alliance?
Ultimately, there are only two positions you can take. If you think the Alliance-Horde conflict should be morally grey, then the extermination of Horde races would be good as it would show this moral ambiguity, an example that the Alliance is not pure good.
And if you think the Alliance-Horde conflict is black-and-white, and the Horde is irredeemably evil, then exterminating its races would not be any less moral than exterminating the black dragons during Cataclysm. We did kill countless black dragon whelps in Blackrock - why would that be different from slaughtering orc children? It’s all for a good cause.