Is the argument that we should try to do things that will benefit our theoretical and theoretically multitudinous descendants? Or is it that just taking action to make their existence more likely is a moral good? Because the latter is just brain dead.

Good question. I think it has to be the latter, given the immense time involved. You can make a connection between driving progress in certain areas today and increasing the odds that humanity eventually colonizes the stars. I don’t think you can make any connection with how well off those far-future humans will be.

If that's what's meant, it's a hilarious perversion of utilitarianism.