My brief opinion:
I believe that prioritizing overall lifespan may not be the most crucial factor to consider. In my view, prioritizing the rescue of an adult holds greater significance, because they have future-oriented interests, a quality that infants lack. Additionally, adults are self-aware and likely have a well-established network of meaningful human connections, including friends and relatives, who would suffer greatly in the event of their loss. In contrast, an infant lacks self-awareness, future-oriented interests, and deep personal relationships.
I do not find the potential argument convincing. It resembles the stance that deems abortion wrong based on the fetus's potential to develop into a fully self-conscious human being, but I think that what actually is (the woman's interests or the actual adult) is so much more important than what could be (the future person).
I'm not so sure that the total amount and aggregation of "happiness" would be increased if the baby is saved.
It's probably true that saving the baby would generate more happiness, because they will be happy for more time than the adult would have been.
However, I think it's also true that allowing the adult to perish could result in significantly more suffering due to all the people that have a personal relationship with them.
I also think that avoiding suffering is much more important then creating happiness. So, while I'm happy to create happiness, I am hesitant to trade the expected average happiness of a future person for the suffering of all the adult's friends and relatives.