Consider this the next time you see an article about declining birth rates.

Yes, but the UN is a body made up of member nations and only has power insofar as the member nations have conviction and capacity to back it up. When the UN judgements don't have teeth or statements don't have follow through, it's because the members don't have conviction or those with conviction don't have enough power. Nonetheless, having a forum where statements can be made that later reveal which members are disingenuous has value. The declarations give people and governments benchmarks to judge those who don't hold to them, and while that doesn't necessarily force them to change it does become part of the record they get evaluated on in all future relations. Everyone understanding each other better is valuable even if it doesn't solve all the problems.