This is not a "Tolkin told you so" situation.
D&D is a brand an in this brand it has been established that elder brains while strong are still mortals and can be killed by other mortals without needing an Illithid.
Especially the Gith are very good at that and are one reason why most brains are in hiding.
But you ignore the power of the nethercrown. That is a "Tolkien told you so" situation. The emperor says that it is no longer an elder brain, but a netherbrain. Still mortal, but untouchable by creatues that do not match the psychic acumen and foresight of a mindflayer. Anyways, the player is the ultimate decider.
It will also be interesting to see if and what BG3 novelties will make it into DND lore.
