You're kind of oversimplifying a lot of things and making strong (potentially unfounded) assumptions. "chronic ingestion of excess carbohydrates" can't be the cause because plenty of people do that and do not get diabetes, therefore it is just a contributing factor. We do not know exactly what goes wrong and where to lead to this condition.
For a more comprehensive source, try reading:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7503727/