If you think about the future of humanity as like the life of a person, then Longtermism starts to look really obvious. Of course the stuff after the first few seconds would matter more than the first few seconds. Of course the first five months matter less than what comes later.
What could a longtermist newborn who is a few seconds old do differently from a random newborn with the goal of maximising their expected future welfare? I see this as the relevant question in your analogy, and I believe there is nothing the longtermist newborn could do differently. Regardless of their goals, it would make sense for them to initially focus on themselves and their close family. From my perspective, the lower the level of foresight, the more it makes sense to focus on increasing the welfare of beings who are closer in time, space, or motivations, even if the ultimate goal is increasing the future welfare of the whole universe. I think humans currently have enough foresight to try to increase the welfare of all animals. However, I am sceptical humans currently have the ability to predict what will increase the welfare of the universe by the time their descendents (biological or not) have 10^50 QALY of total welfare.
What could a longtermist newborn who is a few seconds old do differently from a random newborn with the goal of maximising their expected future welfare? I see this as the relevant question in your analogy, and I believe there is nothing the longtermist newborn could do differently. Regardless of their goals, it would make sense for them to initially focus on themselves and their close family. From my perspective, the lower the level of foresight, the more it makes sense to focus on increasing the welfare of beings who are closer in time, space, or motivations, even if the ultimate goal is increasing the future welfare of the whole universe. I think humans currently have enough foresight to try to increase the welfare of all animals. However, I am sceptical humans currently have the ability to predict what will increase the welfare of the universe by the time their descendents (biological or not) have 10^50 QALY of total welfare.