Well we'll just have to wait and see what Strange Aeons gives us
According to the campaign outline:Lowls will complete the flying polyps’ original goal of bridging the gap between this world and Carcosa. Not only will Thrushmoor fall to the parasite city’s hunger, but Golarion itself could become the latest banquet for a ravenous Great Old One (Xhamen-Dor) returned from death!
So yeah, now Ustlav not only has the Whispering Tyrant, It has a Great old one and Carcosa. Which I mean, not a bad place to put those if you want a buffer state between the Runelords and the Worldwound. Great Old Ones are on par with Demon Lords as far as CR goes, Xin and Karzoug could conceivably take one out working together, and With a Nation of Giants under the sway of Karzoug's Rune Giants, it shouldn't be that hard to hold the line.
It's probably a coin flip as to whether being eaten by Carcosa is better than being eaten by the Worldwound for Thrushmoor though.