On two occasions in the past thirteen months, Blizzard was directly asked about playable High Elves as an Allied race. On both occasions, in an official capacity, they were ruled out on the grounds that Blood Elves ARE High Elves and that High Elves are already playable.
Recently at Blizzcon,someone from the pro High Elf discord approached Ion and asked him if High Elves were possible. I found this image on the Vulpera discord and I found it very illuminating.
Ion's response was pretty much the same as it was on both other occasions in that he recognised there was a passionate community (we know he knew this as in the April Q and A he joked about getting a lot of angry responses from that community). He said they weren't happening in BFA (also already known, as Void Elves would not have been developed if High Elves were planned). Most importantly though, he says they aren't ruled out forever.
WHICH IS EXACTLY WHAT HE SAID IN THE APRIL Q AND A that 'anything is possible in the future'.
It's hypothetically possible and you can't say with certainty that a choice that may not be yours to make a long time down the road will or will not be taken, but at the moment, right now, we have reinforcement upon reinforcement of their antipathy towards playable Alliance High Elves (personified in Void Elves being a thing) means their addition is very, very unlikely in the short to medium term.
Then you couple that with the fact that the cadence of Allied race additions is going to be drastically scaled back post BFA and you will enter a situation where every new Allied Race added to the game is going to a once in every two or even four year opportunity to add something new to the game. I find it very hard to believe that in such a situation, they are going to give the Alliance an effective duplicate of an existing race...particularly when the Alliance has a 95% match to that race in the form of the Void Elves. More pertinent than Ion's third restatement of their position (although the lack of snark seems to have made High Elf fans think it's possible again sadly, as their raised expectations will surely be dashed a few more times with predictable results) was Alex Afrasiabi's response to the Lost Codex question regarding Void Elf customization. In the preamble to that question, the questioner stated his opinion that Void Elves are the High Elf equivalent for the Alliance and Afrasiabi was nodding along in agreement and didn't challenge that.
Long story short, if Blizzard comes out and says something that actually supports playable High Elves being a possibility, I will take note of that.
So what if they do change their minds? That's their perogative. And it will reshape this debate if they do. But they haven't and they show no signs of doing so. But stating that 'anything is possible in future' is not stating it is a possibility, it is stating that at some point someone (not necessarily them) may change their minds. I will give no credence to a hypothetical scenario that does not exist, nor will I discount what Blizzard says on the grounds they may change their minds because that can literally be used to discredit anything they say on any topic if you don't like the answer.