Eating large amounts of carbs is not good if you already have diabetes or are prediabetic. But otherwise, whole grain carbs are very healthy. Type-2 diabetes is not caused by eating carbs, but by eating fat. This is explained in detail here. As pointed out in this lecture, countries like Japan where people used to eat large amounts of carbs used to have low rates of diabetes, and that changed when they started to eat less carbs and more fat.
Fat consumption at levels considered to be normal in the West is not only implicated in diabetes, it also causes cardiovascular disease. This was first rigorously established in the late 1950s, see also here for an article on this subject by the first author written more than 50 years later.
A more recent research result:
...the Tsimane, a forager-horticulturalist population of the Bolivian Amazon with few coronary artery disease risk factors, have the lowest reported levels of coronary artery disease of any population recorded to date.
And note how similar the diets of the Tsimane and the Africans in Uganda are, e.g. they both get less than 20% of their calories from fat.
In general, it's not good to argue on the basis of X is bad because it will cause adverse health effects to people suffering from disease Y. Take e.g. X to be strenuous exercise and Y to be heart disease. Exercise can lower the risk of heart disease, but once you got heart disease, the exercise intensity may need to be limited.
