If contact is made by an alien intelligence, it is very likely to end up being entirely on the alien's terms. Even if the aliens pursued a hands-off approach (or relevant-manipulator-off-approach as the case may be) and allowed us to duke out who represented Earth, it would be solely because they chose that path, not because we had any say in it. Maybe in another couple hundred years that won't be true, but it's likely to stay true for longer than the UN is going to still be an entity.