I would take longtermists seriously if they were one of those poor people they talk so much about. But right now, there is an obvious conflict of interest when they speak about longtermism.
However, these people are proposing a "new axiom set" of sorts, where lives now have the same utility as the possibility of saving life in the future. Ethical principles have no further premises and are derived entirely from one's ideas and feelings. The ideas and feelings here are obviously biased. I think we should look critically at ideas that spring up from biased people.