The reason you’d want to help someone else shouldn’t change if I make you aware of some additional people somewhere which you’re not capable of helping.
Interestingly, Eliezer claims here that that is precisely what caused the change in his case:
If my memory serves me, I converted to average utilitarianism as a direct result of believing in a Big World.
That’s from more than ten years ago. I’m unaware if that is still his position.
I know that this is probably about clearly illustrating the emotional impetus behind one viewpoint, but I can’t get on board with people going “fuck that shit” at difficult tradeoffs.