I appreciate the sentiment here, but this whole thing feels like it's written from a place of privilege I want to name. Your brother found community in his team during hard times - great. But a lot of people don't have the luxury of picking and choosing which tribal affiliations matter and which are just aesthetic. For people navigating racism, religious discrimination, class barriers, sexuality - tribalism isn't an optional identity prop. It's survival.
You're basically saying 'tribalism is fine as long as we all remember we're individuals underneath it.' That works great when your tribe is a sports team and everyone involved has their basic needs met. But take that logic to actual tribal/ethnic/religious divides and it starts to crack. Sometimes the tribe *does* matter more than the relationship, because the tribe is literally protecting you.
I'm glad you and your brother worked it out. I am. But I'm wary of using that resolution as a template for understanding deeper divides. Your Eagles/Cowboys split is resolvable because at the end of the day, neither of you is threatened by the other's fandom. That's not true for a lot of people trying to bridge gaps.
Maybe the real insight is: tribalism is necessary *and* potentially dangerous, depending entirely on context. Not: tribalism is fine as long as we remember it's not real.