The thing is I don't buy that all or even most the high elves are that much holier than thou. As far as we know, the elves that were exiled by Lor'themar resided in Quel'Lithien, those I definitely don't see go void, yet they are no longer with us.
Yet of all the remaining high elves in the alliance, I think they mostly stayed in there because they had already "left" during the second war. I think the main reason high elves exist in the alliance is because they already had a place there and just didn't want to return to a fallen kingdom. Overall I think it's a mix of a lot of issues, but for example, with the Silver Covenant, they are certainly more defined by their hatred for the horde than anything else at this point.
Honestly at this point any High Elf still in the alliance that would be okay with returning to the horde any any circumstance, would have already done so. Any High Elves that would rather be neutral, already are in Dalaran -and if more left over the void elf issue, they would go there-
I think that I mostly agree with you, but I don't see the High Elves that have remained on the alliance for all this time leaving over Void Elves, I just don't think the High Elves specially SC ones are that righteous, they are just self righteous. And at this point I see them more likely to go void to get back at the horde and the blood elves, than go to the horde and the blood elves because of the void.
I think that Alleria herself is kinda the barometer of this for me, I just can't much believe the remaining high elves under her sister Vereesa would shun her and her methods, and I find more likely they would become game for it, and in a more believable introduction the void elves would be the newly organized SC and blood elves exiles.