How many NATO/ISAF have died in Afghanistan? maybe 3-4,000
50,000 Americans were killed in Vietnam.
It isn't near the stage that Vietnam is at.
They cannot win in Afghanistan, that is a given but what they can do though is stabilize the place, build up the ANA, build up government, give the people schools, hospitals ect.. and make the place safer.
After the NATO/ISAF troops pull out the ANA can try and win against the Taliban.
If we were to leave now, every one of those lifes would be all in vain, the thousands of civillians lifes would be in vain and the Taliban/al Qaeda/ Mujahadeen would have a place to train, sell their drugs to buy guns, brainwash and oppress the people of Afghanistan.