I assumed the brain-overthrowing crew (plus brain) would optimistically get to wreak havoc for about 3 months... before being utterly annihalted by anyone with a stake in the proceedings, and then some. There's still that whole army marching towards Baldur's Gate. Made out of figures that paint it as threat to all neighbouring countries, nevermind revenge.
If you're roleplaying someone smart and evil, it doesn't seem like an appealing ending. The crimes involved are much too high profile.
That is practically the only feasible outcome should you decide to linger around with the forces you have available on the Sword Coast and wait for the inevitable, yes. As stated though, it's also definitely not what someone smart and evil, much less an Illithid overlord of a Netherbrain 'forseeing an infinity of possibilities' would do.
You first order of business after decimating Baldur's Gate and sacking it for all its worth in brains and resources is to immediately GTFO the Prime Material with a 'tactical retreat' back into Illithid-controlled space, where you will have to take the role of the figurative Illithid-Caesar and attempt to consolidate the Illithid species itself under your rule - a process which may take years, decades or even centuries prior to executing a true strategic invasion of the Prime ( Which may also be preceded by even more decades of covert subversion and infiltration )
Should you succeed, then the stakeholders will now be forced to contend with potentially millions or more of Illithids, and all of their thralls, under the leadership of the Netherbrain, as opposed to just the Netherbrain, not to mention any alliances you might be able to wrangle out of the Far Realm.
Therefore once again, the real question is whether or not the rest of the Elder Brains would ever agree to support the Netherbrain and its usurpers in order to fulfill the Grand Design more swiftly, and if the Netherbrain could possibly be strong enough to dominate all of them at once should they refuse.
