I agree with @anska on the dialogues. I did like Lae'zel new ending quite a bit. With her making diplomatic inroads with the Githzeri it sounds like: a) she has really grown as character. She even complimented me for teaching her the value of diplomacy b) it sounds like the new society will be militaristic and rigid. But more humane. And that's nice.

But, like the OP, I don't want to think that Lae'zel new empire will be destroyed by the illithids. There are other elder brains out there - including one that was probably the model for the netherbrain. A Netherese arcanist who became an elder brain . . .

For all the horrors of Vlaakith's Gith, for all innocents killed in the monastery, the Gith are the force preventing the rise of the new illthid empire.

More than anything I want a no illthid solution to the end game but if I can't have have Orpheus should be able to turn over his power to Lae'zel. That could either be Orpheus' last act before suicide or Orpheus could decide to step down from office and become Lae'zel's seneschal much like Withers did when he game up his position as the lord of murder, tyranny and the undead.