A false equivalence. The Pandaren were by far the most asked for race to be added. They were a brand new model, brand new culture, and they added something new and fresh to the game we hadn't see before.
Blizzard attempting to re-package an existing race for the other side will provoke a backlash substantially greater than the one the Pandaren endured. The Backlash will be based around 'Blizzard are cheap giving us an existing race' and 'Oh dear god not more damn Elves'. You have to admit there is a point there. Each faction already has an elf race. Why do they need add a second elf option where none is required?
Answer me this. Why should Blizzard make the most popular race of the Horde faction available to Alliance players, simply because those Alliance players don't want to play Horde. If they made that decision, why bother have racial divisions between the factions at all, why not allow every player to choose their faction regardless of race OR story.
In fact, taken to it's logical extreme, why not remove the Alliance and the Horde altogether?
That sort of consequence, the logical end-point of what you are asking for. Which is why I am so vehemently against it. The Pandaren are the exception to the rule, with good sound reasoning as to why they are the exception.
The High Elves are just an exercise in wish-fulfillment that really does begin to break down Red-Blue divisions.
And I'm here to argue against that first step ever being taken.