I agree that tribalism probably was one factor that allowed humans to survive to reach the modern era. However, that doesn't mean that it is still a desirable survival trait now that we're in that era. The world is vastly more populated than it was 30,000 years, or even 100 years, ago and the tribes are bigger and have more opportunities for conflict now.
Nationalism is responsible for some of the most technologically advanced societies in recent history (the U.S. and Great Britain). Nationalism is a powerful force for motivating people across class divisions to work together to build things.
