That's kind of what I was thinking -- illithids want to turn off the sun for their own comfort. Llymic's 'freezing realm of darkness and madness' might be a bit much for their ordered lifestyle, and if the surface-dwellers are converted into Brood Spawn, they've got a serious food problem; their cities don't carry enough slaves and thralls to feed them well. They need a healthy surface population to raid. Just because it'll work doesn't mean it's a good plan. Ah well; it was a neat idea while it lasted.
I think in the past you've said that in your played version of the Forgotten Realms setting, the Chosen are depowered; can you elaborate on that a bit, or point me to a place where you've done so before?