I think that's the point. I don't think assimilation was the goal. You can just see why people want immigration. It's purpose is to create ethnic blocs with distinct cultures. The West kind of built up this gigantic resistance to itself, I suspect, because of Nazism (or the reaction to it in anti-Nazism/anti-fascism to be precise). I think the belief is that Nazism is an inevitable form of the west that has to be stopped with intervention by the state. I think that's evil and don't believe that myself. But clearly a lot of people do. Around the time of JFK's victory, immigration was also viewed as an asset for the growth of urban politics. So immigration is a tool used by a postwar elite to maintain power.