Historically, the Western world has been known for individuality. By contrast, much of the developing world embraces collectivism. Most places tend to have aspects of individuality and collectivism built into identify formation, but the ideal that is pursued tends to be on opposite ends of the spectrum depending on where you are. I think part of what is frustrating people in the Western world about immigration is the shift from individuality to collectivism at the national political level among all parties.
In the past, many immigrants to America wanted to pursue their own individual dream. So people who valued individualism tended to self select and move to America. Over time, the culture in America has shifted along with those of the people in the direction of collectivism. There is no right answer when it comes to identify formation whether someone should focus more on individual definitions or collective ones. But there are usually right answers for an individual, whether they prefer to define themselves in isolation or as part of a group. I suspect that some of the frustration people feel with the way the political landscape is changing is driven by this incompatibility between the average and their preference, and that applies to every party and political belief system.
Ideally, everyone would have a place where you can be more individual or more collectivist depending on your preference. I feel however that in general people might be better off with more collectivism on a micro scale and more individuality on a macro scale. We need lots of people with extreme views counter balancing one another, which requires macro individuality. Likewise, people feel lonely and disconnected despite frictionless networking, which can be addressed by having a collective of people around you that share many overlapping values and perspectives.
Things do not have to be monocultural at a national level for people to feel like they can have extreme views that suit them and for those extreme views to overlap with the people around them. Projecting those extreme views on others at a national level in the name of nostalgia (e.g. Make America Great Again) or impractical idealism (e.g. Pluralism) will not work. That will just create more political violence as people try to seek out the “one true path” forward. There is no one true path, you just have to be tolerant of the existence of people with opposing views.
I suspect that countries will continue to subdivide as people continue to self select into more monocultural groups on a micro basis (e.g. individual states and provinces with strongly held beliefs) and more diverse on a macro basis (e.g. various value systems co-existing within a country or geographic region). The world seems to be absorbing the positive parts of other cultures while shirking the negative parts that hinder progress. We can only move forward if progress is viewed through a lens that allows for macro diversity and micro individuality. Otherwise it will be a zero-sum contests between different types of people, which seems likely to fail.
If you are someone who feels that things are shifting too quickly in the direction of collectivism, it is possible still to do things that allow you to define your level of agency and socialization for yourself. Likewise, if you feel like people are disconnected from each other and do not share enough cultural values, there are various places that are much more monocultural in nature. Seek out diversity among, not necessarily within.