The exponential growth curve of demand is part of how we ensure that the traits system does not become overly abused. It's already possible to take an almost full on culture growth strategy as opposed to a tech growth strategy and while I want that to be possible and a valid alternative way to play, much as an espionage growth as a priority strategy could be, we can't let it become so powerful in the game that it strangely completely replaces the standard tech growth strategy. Further, the output of culture just gets stronger and stronger the more you grow, gather, collect cities and so on and with even some slider dedication to culture, the output can be incredible. It MUST get vastly exponentially more expensive as it goes or it becomes far too easily abused. At first its cool and rewarding as its easy to collect the first ones but each is a lot more power, even more later in the game when you've unlocked the 2nd and 3rd tiers. I'm hoping it should only be maybe 1 or 2 3rd tier rewards are possible at most by the end of the entire game or its probably just too much. Even as it stands now I think you can get probably too many by simply adjusting the slider later to absolutely max out. In the late game this would become horrifically powerful in how much culture all your cities in sum total could produce.