I’ve read a number of assessments like this–and while I think they are spot on with failures in military strategy over the last 20 years, they don’t explain the rapid collapse. The rapid collapse was brokered the way deals are often brokered in Afghanistan–tribal leaders who often have family members on both sides of any conflict in Afghanistan, broker meetings, deals get cut. Most of the provinces fell with minimal or sometimes zero bullets being fired.
I read an article from a former journalist with the Times of London who covered the Soviet withdrawal from Afghanistan and he noted that we should have expected this because the same exact thing happened during that withdrawal. Many areas the communist Afghan state would have local commanders (who belong to say, Tribe X) would meet with Mujahadeen from the same tribe, a tribal elder would set up the meeting. They would come to an amicable arrangement and the communist government commander would agree to a transfer of control. Now the communist Afghan government actually had a stronger core than the Ghani government–they fought some pitched battles in some cases and actually persisted for years after the Soviets left, but they actually turned Kabul over peacefully as well.
We made some mistakes in our design of the Afghan National Army, but they had the capacity to probably avoid being overrun, particularly in cities, where insurgents have serious limitations in their ability to wage large scale assaults against entrenched regular military. But the Taliban being from there, understood that in Afghanistan “deals can be made” that remove the need for such disastrous thing as urban sieges.