What possible good would have come to the US by staying in Lebanon at that time? This was 1983, with the memories of Vietnam still fresh in the public's memory. There would have been no reason to get involved in a civil war in that part of the world while the Cold War was going on. Reagan's mistake was not in leaving Lebanon, it was in entering in the first place. Staying would have been untenable, as even Israel was forced to withdraw. Trying to stay would have resulted in President Mondale. It is better to bite the bullet and correct a bad decision rather than stubbornly sticking to it, particulary when lives are at stake.
I know you think leaving when, and how, we did actually saved American lives, but I think it led to more American deaths in future years up to and including 9-11. So the "possible good" would be saving future American lives.