You would think they would have taken steps to avoid this outcome, but maybe this was the outcome they wanted?

This is what was always going to happen, I'm pretty sure of it. The only goals were political and contractor grift. As soon as those goals were achieved or the cost was higher than the payout, US forces would pullout.

Afghanistan military and police are filled with people, as has been mentioned by other commenters, who are just trying to get a paycheck for as long as they can. They're not going to die fighting the Taliban unless the Taliban forces them to.