I think the "Repugnancy" comes from picturing a very low but positive quality of life as some kind of dull gray monotone, instead of the usual ups and downs, and then feeling enormous boredom, and then projecting that boredom onto the scenario.

I think a world where I felt a huge amount of awful downs over the course of my life is also pretty darn repugnant. Yes, I'd feel some ups as well, but it seems like a world where a smaller population felt almost no downs is probably better than a larger population with lots of downs.