In any case, I think the current Tourism system has two important properties that are not obvious: It mostly reduces the effective Tourism of somewhat large empires, but not of very large empires. Somewhat large empires with ~10-15 Cities are hit sharply by the Tourism modifier. However, the minimum Tourism modifier you can get for your Cities is 25%. Because of this there is a point where each new City is beneficial for your empire-wide Tourism. For the proposed reduction of 5% per City this is 16 Cities. After this point your Tourism modifier stays constant. I have attached a plot for the effective Tourism output of an empire if every City were to produce 1 Tourism per turn. Note the linear increase for very large empires. Also note that a constant amount of Tourism per City is not realistic until very late in the game. The Tourism modifier from the number of Cities stacks additively with the modifiers from Technologies. The late game is the most important phase for a Culture victory since this is when the output of Culture/Tourism is the highest. However, this is also the phase in which the Tourism modifier from the number of Cities matters the least because it is mitigated by Technologies. I don't remember the exact numbers but after I researched The Internet I had like +35% Tourism in all of my ~30 Cities. Printing Press gives you +15% Tourism I think so that alone boosts the empire-wide Tourism output of very large empires by 60%.