The problem with the UN is its powerlessness, unless the US happens to agree with the UN on something which is rare.
The US (with Australia becoming the slobbering lap dog) has nobbled all attempts to create an international body with real international forums for settling issues and providing international intervention.
Which leaves the world exposed - the US is incapable of being the 'world's policeman' and would only act in its own interests anyway. The UN is wounded, discredited and robbed of its ability to reform and assert itself.
And there is no alternative, no public body to replace the UN, no single country with an interest in peace and global development.
There may be plenty, real and political, to criticise about the UN, but the consequences of destroying the UN are frightening.
Until of course, the United States of Europe and the newly aligned China and Russia overtake the US in world reach and power. It's happening - will it come in time?