You're making HUGE assumptions about the good that would come from staying. Why assume that another 50,000 dead in Viet Nam would have prevented the Pol Pot regime in Cambodia? How exactly would that have worked? If the British had stayed in India after WWII, would the eventual partition have been any less violent?

The British stayed in Iraq for 30 years, and yet we have the present situation. Those folks are fighting battles with 1,200 years of history behind them.

Do we really want to spend 1,000 lives a year and destroy the volunteer army, to referee them?