Use non-standard (AKA infinitesimal) numbers: a dust speck is an infinitesimal; there is a clear (and linear) disutility in increasing number of people with specks in their eyes, but no matter how many of them you sum up, you never achieve a disutility of a single person experiencing torture. Add second order if you want to have it more finely grained.
(Of course, this breaks down if you have an infinite number of people with dust specks. But our intuition breaks down anyway when faced with the infinite).
But even with that scheme, it seems that you could just as easily want to maximize the minimal utility as maximize the sum.
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