But nobody knew what they wanted Afghanistan to be and allowed themselves to be engaged in a war of attrition.
We did have a goal. The building of the nation of Afghanistan into a functioning liberal democracy. Of course Afghanistan barely qualifies as a nation and many (most?) have no desire to live in anything resembling a Western nation-state. Short of a truly brutal, imperialist takeover or the installing of a strongman puppet to do the dirty work for the next 50 years, there's no real chance of transitioning the peoples and their cultures over to our way of things. Americans likely have no stomach for such things, and the state of realpolitik is such that this would be truly untenable on the world stage.
Come on. You can't build a functioning liberal democracy in a place like that while doing zilch to promote even the basics of civil society and liberal norms. (And no, elections and ballot boxes are quite irrelevant; they're simply a red herring. Electoral processes have zero legitimacy to begin with in a non-liberal society.) The whole idea is quite simply preposterous.