If not for faction balance that is.
Now before you go on about Garithos and the Purging of Dalaran, yes those were bad actions on the part of the Alliance. Unforgivable? No.
Lest we forget, the original Horde tried to burn Quel’Thalas to the ground and likely would have succeeded had the Alliance not made a tactical decision to warn them and fight by their side. Because of this, the High Elf people survived to fight another day. And now they’re allies with these Orcs AND Trolls, so it’s hardly crazy to suggest they’d want to return to their old allies.
In fact, they were getting ready to do just that in Mists of Pandaria through secret negotiations with King Wrynn. Unfortunately, Garrosh found out and had his agents leave evidence behind to suggest the Blood Elf people were behind the theft of the Focusing Iris that destroyed Theramore and later the Divine Bell.
Now you could blame Jaina for going a bit overboard here and maybe she did. But she already had proof a Blood Elf had been instrumental in seizing the Focusing Iris and in the midst of her rage and grieving there was now evidence they did it again in Dalaran whilst exploiting the Sunreaver’s neutrality.
It was just a big mess that might have been avoided if both sides had been a bit more transparent with each other. If Varian had told Jaina he was negotiating to bring the BE’s into the Alliance and Theron had done the same with Aethas, they all might have realized they were on the same side and that someone else like Garrosh was trying to exploit the secrecy to destroy any reconciliation.
Had Jaina or Aethas been aware of this, despite her anger and distrust I don’t think the Purge would have happened but would have instead seen her and Aethas tentatively team up to get to the bottom of the Divine Bell’s theft and as a result draw the two factions even closer together in the process.
So yeah, both sides were justifiably mad at each other over the fallout for awhile. However, once they were stranded in Nazjatar due to Sylvanas’ and Azshara’s trickery, Theron realized that the Horde Warchief once again exploited his people and saw him as expendable which led to more cordial relations between him and Genn/Jaina once again.
But the real clincher would have to be all the way back in TBC when Kael’thas ended up betraying his people and trying to use the Sunwell to summon the Legion.
With the Sunwell once again tainted in the aftermath, the Blood Elves once again feared they would have to destroy it as they did after Kel’thuzad’s resurrection. Fortunately though, Velen had claimed the heart of Mu’ru and used its power to not only reignite the Sunwell to it’s former glory but also cleanse it entirely so that the Blood Elf people could rise again and be free of the endless hunger that was close to destroying them.
The generosity Velen demonstrated here cannot be overstated, especially considering the very antagonistic nature of their respective people’s relationship at the beginning of TBC. The Blood Elves in particular were outright trying to kill the Draenei people and took great pleasure in the torture and killing thereof, far more than was necessary.
To be blunt, Velen had every justification to just take the heart of Mu’ru, give Theron the finger and leave his people high and dry without a backwards glance and few would blame him for it given what the Blood Elves repeatedly did to the Draenei leading up to that point.
Instead, he took the higher path and didn’t just give the Blood Elves the means to live but gave them a future as well. And to top it off he didn’t ask a thing for it, it was given freely by a people the Blood Elves had shamefully hounded and killed.
THAT’S selflessness. The Horde might have helped push back the Legion, but the Alliance saved them far beyond what they could have asked for much less deserved.
I would argue this would likely have pushed the Blood Elf people to rejoin their Alliance brethren, and why wouldn’t it? Despite the earlier transgressions from Garithos, the Alliance had once again proven they stood for good and were willing to even stand beside enemies and save them simply because it was the right thing to do.
And while it would have been a bit harder to support them so far up north, we know the Alliance would still take it seriously and do all they could to push back the Scourge. Not only would it give them an important base of power in Lordaeron once again but re-unite them with the surviving High Elf population that would only make their people stronger together.
The BE’s might have owed the Forsaken their initial survival for helping them get into the Horde and use Forsaken troops to hold back the Scourge, but later events would prove this would increasingly become a devil’s bargain by the time of the WotLK as Sylvanas would start leveraging it to force Theron to commit troops to Northrend even though he was adamant about it critically weakening their still fragile borders.
That’s why I personally believe the Blood Elves would have rejoined the Alliance by the beginning of WotLK if not earlier if not for balance reasons concerning the Horde.
