It’s simple, my friends. It’s about to be five years since Void Elves became available, so…
Have a few of these little munchkins running around playing with Void critter pets and you’re all set. Even though it’s been five years, WoW should really look into using Shadowlands for a bit of a time skip on Azeroth, it’d help them with 10.0 in so many ways.
Having said that, I think there needs to be a ‘snag’ in the development process that prevents or halts the current creation of Void Elves through the means Magister Umbric and the others use to transform people. Nothing that would cause concern for those who have already transformed, but enough to where they stop because of an unforeseen hiccup.
Say some really anxious, obnoxious and arrogant elves get tired of waiting for their turn, and they try taking the process on their own. Or, another race learns of the process, and tries it for themselves…but it goes predictably, horribly, wrong (ahem…Void San’layn 
). Or, one of the radical Ethereal sects are enraged at how Void Elves managed to maintain their bodies and minds, and they’ve come steal / destroy the process.