Personally I do not believe that the US sovereignty is threatened significantly by the UN. However if it were to be, the issue must be dealt with by changing how the UN works, not abandoning it. by leaving the UN the US would forfeit it's leadership role and renounce any day we have in the way the organization works and the decisions it makes. We would also lose it's military and sanction support. The Trump administration may not want (or deserve) a representation at the UN, but the American people do.