South Vietnam lasted 3 years after the complete withdrawal of US forces, but only because the North was getting massive infusions of Chinese and Soviet aid even while the US was cutting aid to South Vietnam. Najibullah lasted 3 years after the withdrawal of Soviet troops, but his government fell only after Soviet aid was cut off. Assuming US aid to the Afghan security forces remains at around $5b a year, there’s no reason to think the Afghan government will fall.
https://en.wikipedia.org/wiki/China_in_the_Vietnam_War#Confronting_U.S._escalation
https://www.cbo.gov/system/files?file=2018-10/54219-oco_spending.pdf
The reality of counter-insurgency is that it takes decades, assuming the government is unwilling to engage in mass killings that remove significant numbers of enemy civilians from the chessboard. The Colombian counterinsurgency war has been going on for 50+ years. Morocco fought its Berber rebels for 16 years. The Injuns, of course, gave up their fight only after 300+ years of armed resistance.
https://en.wikipedia.org/wiki/Colombian_conflict
https://en.wikipedia.org/wiki/Western_Sahara_War
https://en.wikipedia.org/wiki/American_Indian_Wars
A conflict often touted as a shining example of British proficiency in counter-insurgency nonetheless lasted 40 years.
https://en.wikipedia.org/wiki/Malayan_Emergency
What the Aghan military needs desperately is a shitload of helicopters of every type. That is the key to attacking the enemy anywhere in the country, supporting friendly forces on the ground or under attack in the cities/towns, delivering supplies, medevacing the wounded/sick, and being a constant threat from above.
This was the major problem in Cambodia. I travelled there as a guest of the government and we had one of 6 helicopters in the country at our disposal, but it was a So. Vietnamese chopper. The Cambodians didn’t have anything worthy of their own. The same for their Air Force.
I saw two French Fougha Magister trainers fly over and I know that had a few T-28’s. The MIG 15’s on Ponchentong airfield couldnt’ fly because they had not part with which to fix them.
The (late 1970) Cambodian Army weapons (including those of their militia, often looked like a walking museum from WW1 (they had the old, very long Enfield/Springfield rifles, a No. Korean machinegun on a bike, assorted rifles and a couple 105’s (Rockford Arsenal, 1942 on the carriage).
We have to bomb the hell out of key Taliban towns and some parts of cities they hold, just to let them know what the cost of the war will be on their people (screw collateral damage). They need to be buried knee deep in blood now so that they might pull back into a stale-mate like in Iraq.
Otherwise, a major US withdrawal could lead to another major bloodbath like that which followed our leaving SVN (I’m considered somewhat of an expert, or at least a specialist on the “bloodbath theory” for both SVN and Cambodia as my congressional testimonies warned of what would happen if the communists took over in both countries. Sadly I was right, as my mentors Doug Pike and others taught me early on.