Not sure it is doomed to fail, but it won’t be won by military means alone. Logistics goes thru Pakistan, which limits the amount of troops in Afghanistan to the number that can be supplied by air in an emergency. That in turn depends on the number of airports that can handle big cargo jets. I figured about 100,000 troops but I havn’t done that sort of thing in ages and I’m sure the lift available is different from published figures as is the daily consumption. I used 200 Lbs per day, which I suspect is low. Heilos use LOTS of fuel, which really pushes up the tonnage needed. Without the tactical air, the grunts would eventually end up in deep dodo.
I think it would be better to get all the non-combatant nations out. We still have to supply them, even when they aren’t fighting. Not sure how much political value they have, since they give nations like Germany a political fig leaf that prevent them from actually getting involved.

“The majority of people are timid by nature, and that is why they constantly exaggerate danger. All influences on the military leader, therefore, combine to give him a false impression of his opponent’s strength, and from this arises a new source of indecision.”
- Karl von Clausewitz