Is there even any historical basis for the ability of countries to change their primary culture? Religious conversions happened all the time, but I can't recall any nations just deciding to change the identity of their core ethnicity. And it's not like the feature is used that often -- it's used exclusively for gamey tactics to abuse formable nations. Because culture shifting exists, Paradox has to create tons of arbitrary rules about which country is allowed to form another, which are impossible for anyone to keep track of. This feature made sense in a more sandbox-style experience where each country did not have much identity, like EU3, but I'm not sure why it's in EU4 outside of legacy support.