the disutility of additional dust specking to one person in a short period of time probably grows faster than linearly

That's why I used a googolplex people to balance the growth. All else equal, do you disagree with: "A googolplex people dust specked x times during their lifetime without further ill effect is worse than one person dust specked for x*2 times during their lifetime without further ill effect" for the range concerned?

one person getting specked every second of their life is significantly worse than a couple billion people getting specked once.

I agree. I never said it wasn't.

Have to run - will elaborate later.