As you say, this trial is mostly symbolic, because the US is not a signatory to the ICC and nobody is going to march into the US and drag the criminals out. But It has enormous shame value. Also anybody convicted by the ICC will risk arrest in any of the countries, who have signed on. Thus one practical result of any conviction by the ICC is that it does restrict the international travel severely.
As Andrewboston pointed out, they are still dragging people who have committed crimes during WWII before the courts, even today.
To put that into perspective, any body at the age of 21 in 1945, a very low age for making decisions in any capacity, is today 92 years old.
If our present system either is drastically revised (revolution) or collapses within the next 20 years, there is is plenty of time to still hold a lot of convicts responsible, For Afghanistan, Iraq, Libya, Syria and other countries in the Middle East and North Africa