The High Elves have been a race itself for millennia, in fact, the Blood Elves are those who have deviated from the original race, and thus forming a sub-race themselves.
There are
enough differences between the High Elves of the Alliance and the Blood Elves of the Horde: different customs, different psychologies, different ways of doing things, and even, we could talk about certain physical differences ... (not so natural, but rather artificial ones: as showed
here).
Moreover, even if they were exactly the same race in every way, the High Elves would still have a special place as a race in the ranks of the Alliance.
The High Elves still persist, and are a vital part of the Alliance since the times of Arathor.
The loyalty of the High Elves for the Alliance is commendable: they self-exiled from their ancestral lands in order not to betray the Alliance. The king Anasterian himself (personal friend of the Menethil dynasty for generations) would turn in his grave watching his people becoming addicted to the magic and allying with the Horde (allies of the trolls and enemies of the humans).
Does the fact that they are similar to the Blood Elves makes that High Elves cannot be playable? Of course not, look at the Pandaren (first neutral race present in the Horde and the Alliance)
Are the High Elves too few to be playable? Of course not, look at the Pandaren (how many Pandaren can fit on the back of a giant turtle?) or the Goblins (how many goblins can fit on a boat and survive a shipwreck?)
The High Elves
are ALREADY a race of Alliance, just have not been given the importance and recognition they deserve.
The High Elves in the Alliance are the symmetric counterpart to the Horde Ogres: they complete the quota of historic races on both sides. If Ogres become playable for the Horde (which I personally wish pleasantly),
the High Elves should be also playable for the Alliance.
Are there many elven races in the game? It is possible, but is that bad? Of course not: it gives
more diversity to the game yet. You remember: the trolls are the ancestors of the elves, the Nagas are elves and even the Worgen are here thanks to the intervention of the elves. Is that the “World of Elvencraft”? Not necessarily, humans are in fact the most prolific race of Azeroth and have many subspecies and ancestors.
In fact, in most fantasy worlds there are many elven races, and in the Warcraft universe currently we only have three: the high elves, who are the characteristic formal and honorable allies of humans, the night elves, who are the characteristic independent dark elves, and the blood elves, the characteristic libertines and arrogant elves.
Now the High Elves ingame are mere placeholders (using the blood elf models with blue lenses and the night elf voices)
The High Elves need more love from Blizzard: new models and animations (distinguishable from the Blood Elves), starting zone (Greenwood), a charismatic leader who will gather them all under a common goal (Alleria Windrunner is the perfect candidate ) and increased participation in world events (the silver unicorn banner will wave among the ranks of the Alliance) Glory to the Alliance! For the Quel'dorei!