You have a gift for stating the obvious. The General Assembly is made up of a large number of tin-pot dictatorships whose people didn't elect their representatives in the first place. The Security Council is stymied by France, Russia and China.
No one is saying we are going it alone, simply that US interests are no longer served by the UN, which is not just a little bit corrupt but corrupt from the head (Kofi and Kojo Annan) to the toe (UN peacekeepers in Rwanda). We can accomplish a lot more through bilateral efforts and NATO.
The UN has outlived its usefulness.