I would argue otherwise. The Silver Covenant was in game 12 years ago, out of 15. That’s a long time for a faction to be hanging around. The Silver Covenant was also the de facto remnants of the High Elves that not only chose to not be identified with Blood Elves, but protested the Blood Elves joining the Horde. Then, in Cata, they were complicit in driving the Blood Elves out of Dalaran and cementing them as “villains” in the eyes of the Horde. They were always an Alliance-only faction, tied to the Alliance by Jaina. The High Elves of the Silver Covenant have more ties to the Alliance than even the Ku’Tiran (brand new) or Dark Iron (enemies of all the factions until, maybe Mists).
Using the Silver Covenant would have continued this story. renewed anger at the Horde for what was done to Theramore. The return of Alleria, and Alleria’s bias against the Horde. The idea that allied races are supposed to be smaller sub-sets and not major races. Bringing the Silver Covenant into the Alliance would have been a natural fit. They were never a shared faction (unlike the Highmountain and Nightborne who were) and they have been around for most of World of Warcraft’s timeline - much longer than most of the other allied races.
Compare this with the Void Elves, who not only didn’t exist until the very last second but have a backstory that doesn’t make sense. The Blood Elves are all about toying with powers. It’s why they tapped into the fel. It’s why they allow a warlock class. It’s why they dabbled in the arcane to begin with. Why, suddenly would “void” be an issue except to create an artificial and arbitrary line to not cross in order to justify the concept.
Now, I totally get not having identical elves in both factions, and I share the disappointment that the Nightborne don’t look closer to their NPC silhouette. Personally, I’m more annoyed that this is just another purple elf with silver eyes and we have that look already. Aside from why we have eleventy billion elves in the first place when humans and orcs are supposed to be the two most populated factions.But if we’re going to have eleventy billion elves, it would be nice if Blizzard gave the Alliance some that aren’t yet more blue or purple skins (though I can see this getting solved a bit in Shadowlands). That’s why I thought the worgen change mechanic would be interesting - give the Alliance a race that adds some new visuals to the table, but also make it so you don’t have a simple copy/paste of one race. Blizzard could have also made half-elves if they were coming up with races out of whole cloth - maybe a slightly more slender human shape, with smaller pointed ears and no eye glow.
Ultimately, the reason why void elves are dissatisfying is because there is no reason for them to exist beyond the Alliance needing something. They were introduced, along with Lightsworn against two allied races that both main factions have deep ties to - Blizzard’s meta-narrative notwithstanding. It is quite possible that if Blizzard gave the Alliance Dark Iron first, and used BfA to tell the story of Umbric and his follower’s exile (or set the stage for that story in Legion) more folks would be willing to interpret Void Elves as being High Elves (which they really are).
Personally, I find most of the Alliance Allied races to be un interesting, with Kul’tiran and Dark Iron the hold outs. The Lightsworn and Void Elves are not races but lifestyle choices and their lore is practically nonexistent. And the mechagnomes are…well…an acquired taste at best. Most very much read as the equivalent of Blizzard looking around the room for inspiration and using the first thing that comes in sight. I would not be surprised if Blizzard decided on all the Horde allied races first then realized they needed to do the same for the Alliance and scrambled to shove the first idea that came to mind in order to meet development timetables. And while I do not begrudge the Horde any of their allied races, I really wish Blizzard put as much care and attentiveness to the Alliance analogs.