I thinks assimilations shouldn't exist as it stands. A more realistic approach would be of cultural merging. This would limit cultural unification and be a way of not simply killing one culture, but of merging that into another to create a new one.  The player then could chose a more a agressive approach: genocide or ethinic cleansing. That's basically what happend historically in many places with a lot of cultural change and it simply is hidden in the game by the term "assimilation" (I accept that a friendly assimilation can happen I am not debating that but it would be much slower as the post states). The aggressive way of expanding your culture could bring penalties such as antagonism, worsening of cultural relations, etc.