An unstable Afghanistan is in the benefit of US/India nexus.
A stable Afghanistan is in the benefit of Pakistan.
Why are we fencing our border of Afghanistan and Iran?
The day Afghanistan is stable, everyone knows what we will do.
And BTW in the war on terror, we lost the most perhaps more than Afghanistan.
An unstable Afghanistan maybe in the interest of India...but no longer is it in interest of US(at least not directly).
In case of India...
...for India to turn Afghanistan into a threat against Pak...they can't accomplish it by having an economically broke Afghanistan suffering a civil war...with no sense of a government or stability. Maximum amount of damage they can hope to do with that would be a few terrorist attacks a year at best. Fencing of border, installing sensors, using drones for surveillance, and FC patrols would mitigate most of that.
However these terrorist attacks every now and then from Afghanistan isn't the goal of India...it never was(they can do that kind of stuff from their own border with Pak). The whole time their goal has been to prop up Afghanistan against Pakistan as a country(a fully functioning country)...
...so that it poses enough of a threat for Pak to divert its military to the western front... effectively creating two fronts for Pak. So an unstable Afghanistan wouldn't be of much use to India. The only silver lining would be that if Afghanistan devolves into a civil war...at least Pak backed faction hasn't won.
For India to "succeed" in their Afghan strategy, they would have to make sure that the faction they are backing, wins...in such a significant manner that they can be in power. Only through this scenario does India benefit...and through this US indirectly benefits(by diverting Pak's attention away from India...India would be free to focus on China).
Now consider the spectacular failure of India backed factions over the past few decades. Then consider how much power Afghan Taliban hold on the ground(who are pro Pak). With that u can see how long it will take India to achieve its "best case scenario"...and that's only if everything goes India's way. The scales would have to literally tilt the other way for a while during the civil war.
During this whole time...Afghanistan would continue to serve as the perfect place for extremist anti US militias(like ISIS, Al-Qaeda, etc). So if after spending billions of dollars and thousands of casualties of US soldiers...imagine if there is another terrorist attack in US...
...how would that make the US government look in front of its ppl? How would they be able to call WoT and the end of this Afghan war a victory? How would they justify all the exorbitant costs of all this? Instead of looking stupid...it is more in their direct interest to have a stable Afganistan(preferably not under Iran's influence).
