Even then it would make no sense to ever integrate a culture. The Italic group has 13 culture, lets say you integrate all 12 so you get -12% state culture happiness for the rest of the game. At this point you have about 2000 pops. Every integrated culture is less than 200 pops on it´s own. You expand further, Greece. Africa, Iberia, Illyria. At this point you have let´s say 10-15k pops. The amount of cultures would be: Italic 13, Hellenic 26, Illyrian 9, Iberian 11 + a few gallic,punic and numidian pops. Every culture makes around 2% of your total pops, some a bit less like 1% and other maybe 3-4%. Italics are you culture group so they won´t be very unhappy anyway so why take a stability hit, influence costs and time to integrate 2% of your population for -1% happiness. Even worse with other culture groups. Why integerate 3% of your pops to get -5% for everyone else.
Cultures are just way too small to matter, if theres no way to get a whole group. Oly if you start with a wrong culture it makes sense like bactria, egypt etc.
It would work better if the Happiness bonus and malus is tied to the allowed social standing. If you allow just slaves of that culture they won´t get any happiness bonus an nobody is pissed off. If you make a new culture freeman then they are happier and maybe you get a primary culture happiness malus of 5% for 5-10 years until people got used to it. Same with citizens but harsher and thern for nobles you would get a permament happiness malus.
