High Elves are not an Alliance race first and foremost. In fact they were the last race to join the Alliance and they did so under compulsion. They were also the very first to leave the Alliance. Secondly 'High Elves that became Blood Elves became them with their own history, failings, and successes.' is such an incorrect statement it's hard to know where to begin.
Blood Elves are High Elves. They changed the name. In every other respect they are the High Elves.
They possess the land of Quel'thalas, the city of Silvermoon, the Sunwell, the Farstriders, the Magisters, the Blood Knights and an unbroken lineage of rulers. The Blood Elves did not abandon their past when they renamed themselves, it is those in the Alliance who left their people, not the other way around.
Blood Elves ARE High Elves. This does not hurt their identity because it is a statement of fact and has been stated by the devs on multiple occasions, the most recent being four weeks ago. Pretending that the High Elves in the Alliance are radically different from the Blood Elves is a stance motivated solely by the desire to prove they are different enough to qualify as an Allied race. We have too much in game evidence and word of god evidence from the developers to give that notion credence.
Faction balance is irrelevant in terms of numbers in my opinion. Faction integrity and identity are more important. Granting a duplicate of a core Horde race to the Alliance undermines that integrity, 'blurring the faction lines' as Ion put it.
Issues with the top raiding guilds, which represent a tiny proportion of the playerbase, are not going to be solved by the addition of Alliance High Elves, something which would have an impact on everyone else in the game.
The Horde is already plenty diverse regardless. Everyone has the choice of what to play. Blood Elves are the most popular. Elves are always popular.