I think that's actually a straw man argument. For example, since when did pandering to literal Nazis (i.e. Charlottesville) become a nuanced topic? If r/politics has become more left-leaning in recent times, couldn't it simply be because of things like that? Humans do have tendency to form camps and take sides but sometimes one side really is way off base. I think this is one of those times. The effective leader of the Republican party (the Republican POTUS) continues to demonstrate on a daily basis his lack of basic character and empathy. This is not a nuanced issue and it's trivial to find examples of this.

When one side regularly crosses major red lines, people shouldn't cry foul to both sides when the debate rapidly becomes polarized.

I disagree. A sizable portion of the population very clearly thinks of America as White Country, a perspective that is completely irreconcilable with the liberal melting pot approach.

It's the truth. If reality really is black and white, it's not oversimplifying to acknowledge that.

Modern American politics are not complicated.