That "silly" credibility is often the only thing standing between us and a war. Tell me, do you think the Soviets would have thought twice about unleashing their nuclear arsenal on us if they thought our will to respond (ie our credibility) was weak? How many lives would that cost?
How many lives will a war in China over Taiwan cost because they know we lack the resolve to respond. You want to save lives and prevent needless deaths, then you should make damn sure our credibility is unquestioned when it comes to protecting our interests. That is the often the only thing there is preventing devastating wars.
You seem content to let the world collapse around us and naively believe we won't be effecting. You also only want to get involved if we are directly harmed. But if no one believes our statements to be credible, what exactly is the point? If you are a bully are you going to pick on the kid you perceive as one that will fight back? No. You will pick on the one that you perceive is weak -- ie lacks credibility in their response.
I'm not overly interested in domestic credibility for our leaders, but on the international stage, we better damn well have it, and we better maintain it.