I think you are right, that urbanization weakens tribalism by replacing its allegiances with other forms of social organization that work better in non-rural, nonagrarian economic settings.
Second, I doubt the frequent assertion that urbanization will promote democracy. In America, it is the urban centers that are controlled by machines, bosses and the maffia.
Tribalism is evidence of a lack of central control. You can trace the development of the French state from the time when Hugh Capet only controlled the Ile de Cite to the time of the Sun King who entertained the aristocracy at Versailles through a system of mutual agreements between the king and local vassals.
If you know that story, then you can understand how a national system is being developed similarly in Afghanistan and Iraq and Karsai, Maliki, and Musharref are now negotiating with recalitrant chiefs.