I always feel that if someone says that such-and-such a civ is OP, what that really means is they are fun to play. Rather than nerfing any OP civ, I'd rather see weak civs strengthened a bit to catch up.
I'd strongly disagree that "OP" and "fun" are even remotely identical descriptors. Korea is arguably OP, but they're a solid contender for most boring Civ in the entire game. By contrast, you can do some hilariously fun and stupid stuff with Eleanor and the Khmer, but they're both downright terrible against anyone of a remotely equal skill level.
A theoretical Civ with the bonus of "Doubles all outputs and always wins in combat within owned territory" would be super broken, but would probably not be fun to play as, would absolutely not be fun to play against, and would effectively reduce the number of Civilizations in the game to one. The latter is why some level of balance is important: if there's an echelon of Civilizations that are just objectively better in 99% of scenarios, the Civilizations below that effectively do not exist, and DLC that adds the former sort of Civ effectively removes content from the game.
While the 'ideal' might indeed be "make everyone broken so that nobody is", this isn't really sustainable. Managing 50+ "equally broken" Civilizations would require a level of attention and understanding of the game that it's very apparent nobody on the development team has. Nerfing, if "less fun", is much easier for that sort of developer.
Furthermore, the response to recent Civilizations with "maluses" (such as Mali and the Maori, and more recently the Maya) has been generally very positive (the Maya less than the others, but mostly because they're a V civ trapped in VI). But these maluses are themselves just pre-baked "nerfs"; if nerfs themselves were undesirable, these Civilizations would be much less popular than they ended up being. Why can practices like this not be applied to the present tier of "OP" civs? "It's always been like this and there's no need to change it" isn't a particularly compelling excuse when V and VI already upend a lot of series traditions (for better and for worse).
For example, changing something like "+1 movement to all units" into "+1 movement to all units who start their turn within 9 tiles of a loyal city" results in an ability that is still very powerful, but also presents a new "mini-objective" to pay attention to when actually using it. This would arguably make the Civilization
more fun to play, not less, because one of VI's biggest issues is how boring it can get when you're steamrolling everything. Alternatively, something like "+1 movement to all units, but all units are 25% more expensive until the Industrial Age" would give them a very clear strength and weakness that both the player and their opponents would need to adjust their plans for (GC would want to rush Industrial, everyone else would want to kill them before they reach it.) These are completely arbitrary suggestions and you could take balance in a number of different directions that have nothing to do with these.
. Going the crusade belief route. If I can avoid having Poland wipe out my religion. Seems AI is more aggressive wiping out religion from a recent patch. I'll eventually conquer some coastal cities here soon though.
