In Act 3 of Baldur's Gate 3, if you decide to free Orpheus the Emperor Mindflayer claims that "you leave me no option but to join the Netherbrain".
This seems extremely strange considering that his raison d'etre has always been self-preservation and freedom from the nether brain he was originally enthralled to, which is the entire reason he is helping you destroy it. To prove a point, he even goes as far as killing his best friend Ansur when the latter decided a mercy killing would free him from his fate as an illithid.
Even if there is notable bad blood between both races, Orpheus would know that they'd need an illithid to utilize the netherstones and the Emperor appears to be the logical choice rather than sacrificing someone else.
Why would he do something as out of character as deciding to become its thrall again?