Tens or hundreds of people leaving Quel’thalas every year couldn’t possibly go unnoticed - specially with Blood Elves having a population so small.
Thousands of people living a floating rock in space sounds unfeasible
Alleria introduces us to Void Elves as “a group of Sin’dorei exiled for studying the Void”.
The Void Elf information page simply refers to them as “seeking to harness the corruptive magic of the Void”
We never see anything about political dissent in-game when we see Umbric’s past. All we see is him performing research on the Void with the intention of developing it as a “tool” for the Sin’dorei to use against their enemies. Working years to “comprehend the powers the Void might provide”.
That’s also part of the problem with Void Elves. They’re all about the Void. They’re entirely dedicated to controlling it… and that’s all there is to them.
But this sudden political agenda, with hundreds and hundreds of dissenters from Silvermoon coming to live in a barren rock on space under this cabal looking into the most corrupting force in the cosmos, would come as out of nowhere as the Void Elves themselves did. And that isn’t exactly great.