I've read estimates that manual labourers (medieval and Victorian) probably required around 4,000 calories a day to fuel their labour and maintain body heat in unheated workshops or outside labour.
With metabolisms working at that rate, day in, day out, carbs would have been burned up very quickly - and would have been the only way to get enough calories.
I agree that people have eaten carbs in recent generations. But without all the exercise, and with recent increases in processed foods, sugar, etc, the situation is very different from even 40 years ago.
- added to which, we are the result of several hundred years of dietary changes. Sugar and fruit have become increasingly common. Highly processed bread has become the norm.
If you feed any species poor food over several generations you see progressive declines in health, immunity and a rise in cancer and degenerative disease. They have done this with cats and rats.
Me? I think the problems started with the introduction of farming, thousands of years ago, increased with sugar becoming widely available, with industrialisation and processed foods just adding to it.
My grandparents were already eating badly. Bread, fruit, sugar. They were coasting on the legacy of their ancestors - and they still got T2 on both sides of the family!
Lucky me. I am just the generation when it completely fell apart, and my every dietary choice is damage control for their legacy.