Afghan War is the most unpopular war in American history, which is saying a lot since the competition includes the Iraq and Vietnam wars. Yet there are no street protests, no politicians campaigning against it. Ask anyone and they will tell you it's a worthless waste of resources spent on a worthless patch of land. Still, it continues. The only people who see value in the Afghan War is the professional warrior class. For them it is an opportunity to experiment with technology and tactics and, most importantly, the path to career advancement.
The "all volunteer" army is a misnomer. They are not volunteers but paid professionals. Professionals who need wars in the same ways that auto mechanics need cars and football players need football games. It doesn't matter if the war is objectively stupid, most wars are. They can't be sitting around on they tanks waiting for a morally justified, necessary war. Those are rare; the last for America was World War II. These professionals need to exercise their craft. If the Afghan War did not exist they would need to invent it.
I believe the only thing preventing America going to war with Iran or Syria is the outlet provided for our professional warrior class in Afghanistan. Take that away and the inertia for a replacement war to justify our professional soldiers will become overwhelming. Iran is the likely target, although they may be afraid that Iran will be too able to defend itself and seek an easier victim like Venezuela.
The Founding Fathers were afraid of standing armies and preferred citizen soldiers who would only go to war as needed. Standing armies might be better fighters but their usefulness is outstripped by the dangers they create. Professional soldiers, like ours, tend to separate themselves from the common citizens, living in their own communities and sharing little in common with the nation the allegedly serve. They view us, at best, as obstacles to a smoothly run war. At worst, they see us as pansies and appeasers who refuse to see the absolute necessity of war and the terrifying dangers of peace.
As for us civilians. We have come to view our professional soldiers as convenient toys for our politicians. They can play at war without bothering the citizenship at large. We see our professional soldiers as faceless cannon fodder who will quietly fight the wars we don't give a shit about and not bother us with the petty details like their lives and deaths. Most important, we want our soldiers to behave professionally and not be distracted by moral questions like citizen soldiers are prone to. We want our soldiers to kill professionally, without conscious.