Imagine you have guests coming over to your house, but you have not been informed about their allergies or food preferences. On good will you make a delicious kebab meal however, you realize that your guests are vegetarian. In this simple (maybe trivial) scenario, I would argue that you do not have obligations to your guest because you did not know what they were like and what were their food preferences. Whether you should have inquired about that is a different question, but the point is this: without knowing who you are dealing with you can not have obligations to that person.
This premise is plausible and we will concede it. However, the looming question is whether indeed we are ignorant about what “the future people will be like or what they will need or desire.” Surely we know very fundamental needs with high probability: they will need air, food, water and shelter. A human being begets a human being. We know the fundamental form of a human. Therefore, we know the fundamental form of the future generations. Sure, they may prefer other types of music or clothes, however, fundamentally there are important similarities that enables us to determine what they will need and what they will be like. Ultimately, the degree of similarity between the dinner hosting and the future generations is weak.