Since 1977, there have been a lot more changes than more carb. More fat, fewer vegies, more sugar, bigger portions, more soda, more meat, way more pills from Big Pharma, more toxins, more processed food, more video games, more sedentary, Less hiking and camping, most young people staying indoors, young people don't ride bikes anymore, etc. Deciding its due to the carb isn't clear. Others blame meat, processed food, sugar, etc.
I just finished listening to the Keto Edge summit. What amazed me is how the plant based and keto docs are now coming to almost a consensus. Low processed carbs, low meat, lots of vegies and herbs, some fruit and berries, some fats like nuts, olives, avocado, coconut, movement and exercise, purpose, community, spirituality. Cycling through big carb days, keto days, and fasting days. They sound almost the same.
The longest "study" I can think of is the Blue Zones. They hardly eat meat. They eat a lot of weeds, vegies, stuff from their gardens. Not a lot of fat. Almost nothing processed. Low stress. If it's processed, THEY process it by chopping it up. It's not done months earlier by a corporate giant thousands of miles away and left in storage. They are connected to the Earth and their communities. Sounds like a good life to me.