GX died? Also, from the charts, Astrum is losing nobles, not gaining any. But those charts are kind of garbage for including adventurers...
And it's totally up to them. Even if they had gotten 5 nobles, it's not like those can keep them accountable. Astrum has the power to do what it wants. They have the most nobles and a bunch of very rich regions pretty strategically placed. It'd take a strong coalition just to push them out of Westfold, and then... stalemate. I can't imagine any realm being able to take any of their cities.