The geographic distribution of the Pashtun according to the above map would not allow for an easy partition of Afghanistan on the basis of ethnicity - the Pashtun appear to live across the South, Central, Eastern and Western Afghanistan.
Secondly, if the argument is that the Taliban can be given control of the Pashtun provinces (and they'll settle for that) and be expected to not support 'global terrorism', then why can't that arrangement be arrived at within a united Afghanistan in the form of a Federation, with the various ethnically divided regions having significant autonomy along the likes of Iraq?
Autonomy for the provinces/regions within one nation would allow for self-government for the various ethnic groups and a respite from ethnic tensions possibly, while also keeping the nation intact and allowing for nationhood and unity amongst the various ethnic groups to grow over the longer term.
In addition, I have seen nothing in terms of Afghan opinion suggesting that there is a desire for partitioning Afghanistan amongst any of the ethnic groups. If anything there is a desire for expansionism, through acquisition of Pakistani territory, amongst some Afghan Pakhtun.