I think having an outgroup is avoidable, but it requires ongoing vigilance, and an awareness that it is easy to slip into building your identity partly by deciding who to hate.
If we're careful I do think it's possible to maintain an attitude that we're basically all in this together, and we may have significant disagreements on all kinds of things, but at the end of the day we're still all humans and worthy of kindness and respect.
If you care about some people more than others, then you have an ingroup and an outgroup. Would you really want to not care for your own family members any more than you care for a random person on the other side of the planet? That’s what striving for no outgroups entails.
I think that’s a horribly misguided mindset. A better approach is to tolerate the outgroups and facilitate live and let live arrangements. Then you can focus more on appreciating your tribe and less on fighting the others.
I was taking outgroup to mean a set of people that we find so abhorrent that they're unworthy of any level of consideration or respect, and that we even wished didn't exist. The term has a pretty specific meaning, and it goes beyond just a group other than our own.
If we're careful I do think it's possible to maintain an attitude that we're basically all in this together, and we may have significant disagreements on all kinds of things, but at the end of the day we're still all humans and worthy of kindness and respect.