"Don’t Overestimate the Taliban"
One of the worst habits of national security thinking is to look at a troubling situation, then ask what would happen if you assume the worst. And then having done that, you ask what would happen if you assume the worst next set of knock-on consequences. Suddenly your initial situation has gone from troubling to “alarming” and nobody notices that the same logic could induce panic if applied to anything. For example, via Michael Cohen, Jari Lindholm writes:
After years of jihad alongside al-Qaeda and other international militants, they would not merely allow terrorist organisations to use Afghanistan as a base; they would encourage it. Naturally, toppling the Pakistani government by supporting their Pashtun brethren would be high on the Taleban to-do list, as they would want to see a friendly, ISI-backed general return to power in Islamabad. In turn, they would gladly help in providing him with the terrorist cannon fodder he would need for his covert operations in India.
And maybe for their next act, they’ll team up with Chechens and invade Russia in the winter!
Seriously, however high “toppling the Pakistani government” may be on the Taliban to-do list, it’s still, you know, hard. Indeed, it’s worth emphasizing that on both sides of the border there are actual human beings living and fighting against the Taliban who would continue to resist Taliban domination of their countries even if the United States didn’t lift a finger to help them. It’s important not to confuse the difficulty the U.S., Karzai, Pakistan, etc. have with dislodging the Pakistan from their home base with the idea that the Taliban is some kind of all-powerful super-army capable of easily overrunning Islamabad or the Panjshir Valley.