Please point out to me anyone who has been arguing for no "specific content" whatsoever. It is only with TAGs that any of us have a problem with.
I'm talking for myself, here, not for the whole "anti mission trees faction", but I am also against unique units, mechanics and national ideas (to not talk about monuments, but that's another conversation).
Let me try to explain.
I am not against the mere existence of things like streltsy or banners, nor am I against the existence of militarization as a possible mechanic. I am also not against the fact that France *could* start with a unique mechanic called "appanages". I am against the idea that those should be linked to cultures. In my mind, they should be government reforms, or things that are unlocked when your country reaches certain pre-requisites.
However, if they are "cultural", my main gripe then becomes that they offer a fixed vision of culture, just like national ideas give a fixed idea of what a country is good at. They go with some tributtons and unique mechanics.
I would say I'm against a fixed view of history. From 1337 onward, the World should be alive and moving. This doesn't preclude having ongoing situations in parts of the world. It doesn't either mean that polities shouldn't start with particular mechanics enabled, but as much as possible, they shouldn't stay stuck with those until 1836.
Now, going back to streltsy, banners and appanages, if it were possible for other countries to get them, it shouldn't be a walk in the park. And making them so difficult to get as to be functionnally only something Russians, Manchus and French can get probably defeats the purpose of making them attainable by others. Why spend so much development time making a mechanic universal if it is universal only in principle?
Yet I think if you study why those realities evolved, you should be able to get an idea of how they could have evolved elsewhere.
And... I'm not talking about the names. A british imperial guard would obviously not be called "streltsy". I'm talking about the reality this mechanic is meant to represent.
As for religions, I also think in an ideal world, they should be reformable. What was the reformation if not an attempt to reform the Catholic Church? However, moreso than with my previous comments, I think making a mechanic by which you could get a Theravada papacy or a Shinto college of cardinals, while fun, would point to processes that would have took a while, or a considerable effort to be put in place.
I like CK3 approach, but I think it's too easy to make massive changes to religion/culture with minimal input from what's happening in-game.