Thanks to all your smart and insightful comments, I've discovered the source of my 80/20 conceptual problem: I'm an idiot. Although many of you were gently pushing me in the right direction, it was Jakob Neilsen (comment 15) who put his finger on it. I'd somehow forgotten that the 80 and 20 are supposed to be percentages of different things.
The conventional definition is that 20% of products make up 80% of revenues. But
when I suggested that this would evolve to a 50-50 rule--that the money
in markets of the future will be roughly balanced between hits and
niches--I'd changed the definition. In the new formulation, both term refer to revenue. Doh!
One of the best bits of advice many of you gave was not to take 80/20 too seriously. Its power is in its universal resonance and familiarity, not the absolute accuracy of its numbers. I'll keep that in mind as I continue work on the chapter and I thank everyone for helping me clear this up.