When fighting insurgents, victory is outlasting them. We don't have to be there to do the fighting, but as long as our enemies are killed and the guys we're backing don't lose, that's winning. It's a lot like Indian-fighting - our forebears fought the Indians for centuries, almost until the dawn of the 20th century, before armed clashes finally ended.
The Brits like to cite the Malayan Emergency (1945-1979) as an instance of a well-executed campaign where British troops showed our guys (later in Vietnam) how counter-insurgency ought to be done. In reality, British participation in the campaign (using a combo of British draftees and regular troops) as a primary combatant lasted 15 years, and British advisers stayed for another couple of decades, into the late 1970's, along with some amount of British military aid. All told, the counter-insurgency campaign lasted almost 35 years, until the Chinese Communists ended their funding, training and supply program for the Malayan Communist Party.
That's the nature of these things - they take decades of whack-a-mole until the dead-enders give up (usually when their funding dries up). For instance, the Colombians have been knocking off leftist guerrillas for almost 50 years, since 1964, and FARC is still alive and kicking.