Kings Island's primarily role in the chain right now is to be a cash cow for the local family demographic, who is the demographic most likely to spend the most money at the park. The park knows it can get away with not installing major new additions because the local demographic will renew their season passes regardless as there's not much competition within the immediate area. Kentucky Kingdom and Holiday World are somewhat close by, but if you're a KI local, you're looking anywhere between 1.5 hours and 3 hours to get to those parks and neither of them are nearly to the same scale as Kings Island.
Cedar Point's role will always be to be "the roller coaster capital of the world", building up an elite collection of coasters and rides to attract people from around the world, and that includes Kings Island's local demographic. Cedar Fair/Six Flags wants to keep the elite record-breaking coasters exclusive to Cedar Point so passholders of Kings Island who want to experience that stuff will need to travel north to do so. (and gives them more incentive to buy the all-park passport which is also very important)
I do think there will come a time when Kings Island will add a new thrill coaster to the park. Likewise, I think there will come a time when Cedar Point will add a new family attraction similar to how they just added Wild Mouse. Just because these parks focus on different demographics doesn't mean they will exclusively add attractions to cater to those different demographics. With the families being the main priority for Kings Island right now, it may be some time before we see that thrill coaster come. In addition, don't expect the coaster to be an elite coaster that outdoes something at Cedar Point.