I don’t think, a linear nerf across the board will be the solution.
Controlling one ore two monopolies will not screw tourism. It’s multiple monopolies that lead to the absurd percentages in the thousands. They come with an extended militaristic play style; players aiming for a peaceful game (and hence more likely those aiming for a Cultural Victory) will never “benefit” from a multitude of monopolies but will be stuck to just a few.
I think it’s important to tackle the tourism issue for conquest players without devaluating the game mechanic completely for peaceful players limiting themselves to a smaller territory.
My proposed solution for this: diminishing returns.
The first two monopolies could both add 100% to tourism each.
The third one would only add 50%.
A fourth one 25%. And so on.
(All numbers just as an example. They could even mean “100% of what the current formula does”)
With such an approach, players aiming for a Conquest Victory will not unintentionally win a Cultural Victory, while those aiming for the later still have an incentive to establish a small amount of monopolies.
Disagree with the bolded claim. In my fast tourism run, I was getting +580% from two monopolies. While technically not a bonus in the thousands, it is plenty enough to totally break the game. Anything giving over 100% tourism to every civ that early into the game is starting to get questionable imo.