Had they "obliterated" everything then the US + Allies (or what allies they might have still got left after said obliteration) would have lost even more on the grand scale of things compared to a remote war in Central Asia. No amount of Hollywood + Western Media white-washing would have managed to get that stink off.
Hasn't the US carpet bombed Vietnam and still lost?
That was the issue with Vietnam (and with Afghanistan in a certain way)
Winning means you installed/recovered democracy.
Both the US and the Soviets lost because Afghanistan isn't a country in a traditional sense; it's just an area of land with _lots_ of regions countrolled by war lords that doesn't have allegiance to anything central (i.e. "the country Afghanistan").
It's impossible to win a war against something like that, unless - as you correctly points out - you obliterate everything.
The Soviet Union did that and also lost. The only way to win in Afghanistan is by fighting in Pakistan. And that means a full invasion, not a half-measure as was tried and failed in Laos and Cambodia.
As horrible as the Afghanistan war was, it would have been even worse if they had just carpet bombed everything. Add robotics, and it quickly enters nightmare fuel worse than the Terminator movies.