Last bit is up in the air seeing as certain race additions (mechagnomes the biggest, but also nightborne/lightforged/highmountain) have had a very lukewarm reception or (in the case of nightborne) have had feedback that the AR itself needs much more work, while some races like brown orcs and blue blood elves had an exciting reception despite being essentially recolor tweaks.
I think what will happen (and I believe Ion even mentioned so) is that the existing AR will at some point receive a brushing up. On top of that, with increased customizations coming the definition of how "new races" and "allied races" get added is also up in the air.
As we see that wildhammer dwarves, who were long suggested as an Allied Race once the system was revealed has been included in base dwarves. I can therefore see Blizzard making some form of High Elves available through Void Elves.
It saves them from, as you say, "spending a valuable opportunity" (which I take here as another race slot) as adding them in similar to wildhammer avoids that. Also follows the goal of their increased customizations coming in Shadowlands (more variety within a race option) and adds it to Allied Races.
Win/win in a sense, not a complete win for those wanting High Elves as a stand-alone Allied Race, but allows an Alliance race member to become playable in a manner (thus conserving the recognizable silhouettes available within a given faction) that doesn't "take up a slot" where new races/ARs are concerned.
Of course all the above also assumes that adding races/race customization is going to stay as the methods introduced as of today and as we have seen within the scope of 1 expansion these methods of adding races/racial customization will most likely change as Blizzard iterates on their ideas.