But if they were leaving themselves space to maybe, one day, make the high elves a playable race on the Alliance, then the perfect moment for that was the addition of allied races at the end of Legion and beginning of BFA. So even if that was their intent, once the opportunity arose to do something with it, not only did they NOT do it, but they went out of their way to rule it out. They even created a brand new variant which essentially ended the debate entirely as they would not have created Void Elves had they ever had any intention of adding high elves to the future. As someone who believes the gameplay reasons for not adding high elves were water-tight before, because of Blood Elves, the addition of Void Elves is the equivalent of putting a second lock on that door.
Mostly because second guessing the developers is a fool's errand. If you want to debate a topic you can only do so with the information you have to hand. Saying 'well they can retcon it' or 'they can change it' is intellectually lazy. At that point debate collapses because it moves from what we know and what we can validate and onto pure wish fulfilment.
At the moment, every single scrap of information and commentary is against the addition of high elves. And this isn't just a one off, it's the cumulative result of fifteen years of developer commentary and storytelling. Of course, there is a difference to retconning a detail from an old manual or a relatively obscure novel based upon the franchise and changing a reality players have lived with for the past decade and a half. The longer an important fact or feature is emphasised in the game or by the developers, the harder it is for them to change course. The high elven exiles fall squarely into this bracket. Time and again we have been told they are a nearly dead, vanished group, the lore rationale that was again cited when they were asked to explain why Void Elves and not High Elves.
Nobody here is a developer. But perhaps the pro High Elf crowd would have more luck using that as a retort if they didn't have a small cottage industry seemingly dedicated to trying to prove the actual developers are wrong. After all, the logical response to a pro High Elfer accusing someone of not being a developer would be to link developer commentary and I guarantee you the response would be a rubbishing of what that dev said.
I was around in classic. And yes, I was well aware that one of the reasons Blood Elves were added to the Horde to resolve the faction imbalance of the time. There were several other reasons too
https://twitter.com/Xelnath/status/1088387488429662208
And yes, the lore was adjusted to accommodate it. Previously it had been believed all Blood Elves had gone to Outland, when in actual fact the vast majority remained behind in Silvermoon. But the lore was also a lot more malleable earlier in the game's lifecycle, with most of the retcons dealing with manuals and novels. They could get away with that with ease. It is far harder to retcon something like the high elves now because retconning a novel or manual is one thing, retconning one of the best known stories of the Warcraft franchise is quite another.
As for High Elves being around for a hypothetical northrend expansion I have seen no evidence of such a suggestion, nor has it ever been brought up. If you have an interview or record proving this I would like to see it. I am not saying you are wrong, only that the passage of time means that memories are unreliable. Something that cannot be independently verified and which goes against the grain of what we know cannot be taken as substantive evidence.
Gameplay will always win over lore. And in the early days, lore was adjusted to support these decisions. But as we are now much deeper into the game, with people experiencing the lore as a living thing rather than something to be read up on, such wide ranging retcons as we knew are no longer possible. From changing the circumstances of entire races over a decade ago, a retcon now deals with smaller stuff (such as the origin of the Helm of the Domination) or with character motivations. But retcons of that original scale are no longer really needed, as the game has moved beyond the realms of the RTS. Everything created anew doesn't need to reconcile lore and gameplay as they are designed with both in mind.
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It really is that different. The entire basis of this discussion is the hypothetical circumstance of a Blood Elf defecting from Silvermoon to become a high elf, the sole motivating factor of which is ideological. This is not just betraying the Horde, it is also betraying Silvermoon.
What happened in Mists of Pandaria was not about individual loyalty, it was about the status of the Kingdom of Quel'thalas at a particular moment in time i.e. the Horde collapsing due to Garrosh's supremacism. The motive was not ideological, as it would be a hypothetical defector, it was one of self-preservation.
And yes, there were some street preachers back in TBC. That particular plotline has not been developed since that moment nearly thirteen years ago. In fact, the last word we had on the sympathies of the Blood Elves was Lor'themar worrying they wouldn't rise up against the Warchief. He would only worry about that if he was concerned that a large proportion of the population was on board with Sylvanas' actions up until that point. Remember, it was not Sylvanas' atrocities that turned the majority of the Horde against her, it was the revelation she had no loyalty to the Horde herself.
While it is hypothetically possible for an individual Blood Elf to defect and become a High Elf exile,we also have zero evidence of this ever occurring since the initial schism. There simply will not have been enough to overcome the lore based objection to high elven playability, their incredibly low population.
The only thing that does seem to motivate some Blood Elves to leave Silvermoon is the offer of forbidden power, which is entirely consistent with how they have operated.