The details in the book excerpt linked to above are telling.

One very troubling idea, but one that might have too much truth to it, is that an outgroup is necessary to generate unity in an ingroup. Donald Yacovone asserts:

… if no slaves ever existed in the South, Northern white theorists, religious leaders, intellectuals, writers, educators, politicians, and lawyers would have invented a lesser race (which is what happened) to build white democratic solidarity, and in that way make democratic culture and political institutions possible. As one of our greatest authors, Toni Morrison, once explained, in the United States the rights of man were “inevitably yoked to Africanism.” In other words, American democracy depended on Black inequality to sustain white equality.

Jonathan Haidt talks about “groupishness” as one mode of being human. Groupishness makes us act very well towards the ingroup and badly toward the outgroup:

As Rob Henderson says, "A psychopath is a person who treats their ingroup the same way that normal people treat their outgroup."

I hope that having an outgroup is only a shortcut to getting ingroup solidarity and not an absolute necessity. It may require an advance in social cohesion technology to have any better way.