A rapid US withdrawal will lead to a rapid Northern alliance collapse, there will be pocket of resistance here and there and the Northern Alliance will retreat to the North no doubt to continue their guerilla operations in the bosom of Hindu-Khush mountains. Pakistan should enforce a no fly zone to ensure no helis or planes can escape from Kabul to help ensure a long lasting peace.
It would be interesting to see if India will rush troops to prop up the tottering Northern alliance as this is NA's last hope to prevent a decisive defeat. Ashraf Ghani's must be contemplating his future and his escape route. He must have memories of what happened to President Najibullah , the torture and then public hanging after the fall of Kabul, a President who was unceremoniously strung up from a concrete traffic-control post at the gates of the presidential palace as thousands watched.
The Taliban are well practiced in the capture of Kabul as they have done it twice before. They will capture the Presidential Palace and the Ministries of Defense, Security and Foreign Affairs buildings and position their soldiers at key points. Taliban soldiers will search Kabul for the last remnants of the NA loyalists flushing them out for summary execution. The Afghan people will be resigned to forced jubilation and those who resist will be punished. Switching sides and loyalties has no price for them, they have endured so much , almost half century of civil wars and invasions.