However I do disagree with the assessment. The ICC lacks credibility because the signatories to this convention themselves lack credibility.
The history of this strange institution demonstrates that very well.
Had the U.S. become a signatory it would likely have somewhat more credibility as you term it. However, I believe it would still be an ineffective organization, lacking in force, credibility and significant beneficial effects. It is merely an artful disguise for an attack on the soverignty of nations by the consenting least common denominator of consenting nations looking to evade their own responsibilities with something resembling Aesops' bell for the cat.
There are all sorts of well-founded charges which could be brought against Colonel Gaddafi's human rights abuses in Libya. It would seem to me that International Criminal Court (if it had the full support of the international community) would serve a powerful & proper function in support of the citizens of that country. That may not put and end to Gaddifi's genocidal madness, of course, but it surely give the international community considerably more united "muscle"?
For some of us the very idea of optimism, hope, idealism etc is not in contradiction to harsh reality but rather a direction we would seek. A utopia is not a place but it can be a goal, changing the world just means changing ourselves to be slightly more humane than we were the day before.
However, in view of all that I know about human history and all that I have learned about the mass behavior of people, I very strongly believe that my skepticism is far more realistic than your optimism.
You're all hat and no cattle.