RAND did a study years ago about COIN campaigns. The average length, win or lose, was 13 years. COIN/ FID works, but it usually fails because the political will isn't there. Cases like El Sal, Oman, and Malaya occur off the public's radar (and before the Internet) and failures are rarely in stealth mode.
COIN/ FID are valuable and achievable, but impractical and unwinnable unless they are secret. At best you can only commit small forces the US public won't care about because of the numbers and lack of media coverage. That means conventional forces MUST keep their grubby paws off of those efforts.
Curiously, all of the above sounds like what the Army's Special Forces used to do.