The individuals who have the largest stakes can change their policy on how they use that stake. That's not an option for me (I can only choose how I apply mine) but it is an option for them.
I don't think anyone ever claimed this. That is not the same thing as saying that incentives don't influence behavior. They certainly do.
When people argue that we don't have a problem with culture, that is essentially what they are saying, that only incentives are a way for behaviour of participants in the system to change.
I'm going to say it again. We don't have a knob we can turn to change the culture. By contrast, we can adjust the code. Perhaps that influences culture but such uncertain influence is the best we can do. Culture is a dependent variable not an independent one.