Great question, Lou, but not much of an answer in terms of research, and I have my doubts there’s any to find.
I feel confident about “broad brushstrokes” regarding nutrition, especially in terms of refined carbs and industrial seed oils, but pros and cons of various kinds of fruit and how many to eat are going to live in the world of individual opinion and controversy, I think.
I’ve heard it said that even “5-a-day” doesn’t actually have an evidence base (i.e. research behind it)! Doubt that it’s harmful, though 🙂
My guess would be that you’re right about overeating fruit at 7/day as a substitute for sugar, but it is my guess to be honest. There certainly are people who would recommend no fruit at all and others who swear by nothing but. Dr Robert Lustig has made an interesting observation that in a pre-industrial world, all fruit was seasonal, to be consumed in quantity to fatten us up for the coming food scarcity over winter.