Empires breaking up tends to be extremely violent. Just look at what happened during the collapse of the European empires.
Also what replaces them is not necessarily better. Not all evil comes from the US. Other countries are perfectly able of horrible stuff as well.
Looking at the countries currently trying to replace the US none of them are better. Russia an Iran are pretty obvious. China is already working hard on enslaving the poorest countries in the world. For most of the rest the US Empire falling apart, just means local dictators continue their rule.
Add to that the violence of the conflicts and I gurantee you very few will celebrate that collapse.