Blizzard is on record as saying that Void Elves are looking for a way to make more.
But right now there is no way to make more Void Elves, at least not the kind that we have playable. Alleria’s methods of harnessing the Void is quite different than the process that transformed Sin’dorei into Ren’dorei and perhaps those methods will be the way forward. We’ll have to wait and see on that. Perhaps it will coincide with more customzation options (even if I find that unlikely).
With that said, it just isn’t plausible to expect that any significant number of sane individuals would voluntarily saddle themselves with maddening whispers for the rest of their lives just to be a better shadow priest. There are far safer paths to power.