Still learning about the tenets of this philosophy here, nor do I have a position on it yet, so might get this wrong... However, isn't there a more charitable argument you could make that looks something like this:

4. Unlike with cavemen and Socrates' time, we now have decisions to make that could lock us out of continued proliferation of human life in the future. As such, the longtermists are bringing attention to those, and trying to help us to make the right decisions, instead of focusing resources on short-term returns.

A good example they might support would be spending more money on space tech and less on food for Africa. Resources aren't limitless, and some aren't renewable at all, so we have to use those wisely to position our heirs such that they could take the next step.

They might also make the slightly more tenuous claim that since we know more about the nature of the universe, there could be some invariants that we'll certainly need to see through this vision (e.g., establishing self-sustaining life off the planet). Those will be true in the future, just as the caveman could have assumed we would still need food and shelter of some kind in present day.