I agree that popping in and out of ketosis will not work well for many people, but I have to believe that evolutionarily speaking, that metabolic flexibility, more than permanent ketosis, is the natural state. And that the number of people it would work for is higher than 15%.
Before agriculture, people ate fruits and honey whenever they could get them. Find a patch of berries and gorge on them. Then when they were gone, back to hunting and gathering, probably in ketosis. This would be more common in summer and autumn. Spring and winter diets would be significantly different in temperate zones, but less so in tropical zones.
For example, Kitavans live on a tropical island and eat root vegetables (yam, sweet potato, taro, tapioca), fruit (banana, papaya, pineapple, mango, guava, watermelon, pumpkin), vegetables, fish and coconuts. They get about 70% of their calories from carbs, yet are lean and healthy. The caveat is that their carbs are not processed and not grains. Okinawans are eat even more carbs. The traditional Okinawan diet was about 80 percent carbohydrates. Before 1940 Okinawans also consumed fish at least three times per week together with seven servings of vegetables and maybe one or two servings of grain per day. They also ate two servings of flavonoid-rich soy, usually in the form of tofu. Perhaps there are genetic variations that explain how these cultures thrive on high-carb diets, but I haven’t seen that anywhere.
The fact that the worldwide percentage of people who are overweight according to BMI in 2014 was less than 40% also suggests to me that the number that a cyclical and seasonal approach would work for would be significantly higher than 15%. How much higher, if at all, I have no idea. I think the number would probably be fairly low in areas that have the highest obesity rates and higher in areas with low obesity rates, assuming that obesity rate is correlated with insulin resistance.
Just my 2 cents.