Of course the problem is that if you separated a nation out of Afghanistan based on Pashtun ethnicity, they would want their piece of Pakistan to go with it - and so it would go. There's hardly any end to the story once you start subdividing enclaves. The curious thing is that in Europe, centuries of warfare eventually did force ethnic separation, so by the 20th century things had calmed down in terms of ethnic wars (other than Yugoslavia) - which gave them the luxury of having ideological ones. (). In areas where the Ottomans or Mongols ruled, they had a different system where you had parallel ruling jurisdictions operating for different religious groups in the same geography. But once those old poly jurisdictional regimes collapsed, it left all kinds of people who didn't want to be ruled by each other in highly mixed geographies and the fallout still drives a lot of the conflict in places like Iraq, Syria, Lebanon, and to a degree Afghanistan.