That's a good point. Invading Gaul et al. was probably more lucrative than invading a country like Afghanistan as the Romans were more obviously and unabashedly imperialistic and that war had to do with territorial expansion.

The invasion of Gaul is more akin to the way (relatively) early Americans forced out the Native Americans, and their culture certainly changed/doesn't really exist anymore. WWII Japan and Nazi Germany are examples of a places where a short war changed the culture.

The reason it won't work any more is because that style of 'total war' appears to have gone out of style. Probably a good thing, though Fareed Zakaria was on the Daily Show the other night talking about how one reason America has been so powerful is because Europe was leveled in WWII and the US had the industry to rebuild.