But the cost to Afghanistan has been enormous. Four decades of constant war, millions dead, and near the lowest lifespan and living standard in the world.
Is that really the defense strategy a country should aim for ? Will a modern latte sipping atheist nation really execute it?
Also, there are many other reasons to invade countries than to hold them long term.
In a war, your losses don't usually concern your enemy. But they do care about how much it costs them to achieve their goals, so if you can make it prohibitively expensive, you have a sound defense.
The goal was to root out what they perceived as an enemy and hostile nation that harbored terrorists, and then recreate a society much more tolerant and sympathetic toward Western values. In that, there is no conquering needed. It's called nation building, and it's very different.
I put it in quotes not because I believe in moral relativism, but because in the conflict against the USSR, official US policy was that the mujahideen were "good guys, or at least not-as-bad-as-the-other-guys," while in our own invasion, they were bad guys, an assessment I share due to their harboring of terrorists who intentionally murdered thousands of civilians.