Taliban and S-400 ?? I doubt they could operate them. On a side note it would be easy for Delta Force to raid Afghanistan and seize S-400 for inspection,So Americans played Taliban like a riddle. They wanted a secure passage to get out with all their precious equipment and cargo, they knew that if we do not make a peace with Taliban before leaving, Taliban will hit us more severely like they did to USSR when it was leaving Afghanistan back in 1989.
Trust and Pashtunwali makes Taliban so venerable to 21st Century treacherous wars, they really need People who can deal with American hypocrisy. Taliban should have known that the American will never honor their side of the bargain after what happened to Iran's Nuclear deal.
However, I'm still quite sure that after sometime Taliban will eventually take over all of the Afghanistan territory, but running the state affairs after that is going to be a huge challenge for Taliban, their is one thing though, the day when Taliban come into power I reckon Taliban will request Russia for acquiring s400 system.