The costs are a potential outcry from existing BElf players. The benefits are increased happiness from players - Horde and Alliance - who want HElfs, more economical and effective story development that -like pandas - crosses both factions, increased visual diversity at minimal cost and effort and rounding out the faction so it can be considered whole and complete.You're running on pure hope and delusion, that's all it is. Any cold, rational analysis of the issue shows that the costs of implementing High Elves are not worth the reward.
It's a pure judgement call as to which way is best. That you feel HElfs are better not added to the game does not mean Blizzard won't feel differently.