I don't know how big you are, thatbags, but you should probably be eating about 200-250g of carbs every day, which will give you 800-1000 calories or about half of your energy requirement. All carbs eventually break down to glucose, so you want to choose the ones which will take longest to break down eg. not simple sugars, although they won't kill you, if eaten in moderation. I bet your grandmother knew that. Fat slows down the digestion of carbs. It's always amazed me that traditional foods, such as cheese on toast, a bit of butter on a baked potato, etc. are reasonably healthy options without people knowing anything at all about nutrition.
I agree with you about not needing to know much about the science of nutrition to eat healthily enough. I don't understand why the supplements and 'superfoods' market is worth millions when the simple truth, as you say, is moderation in all things.
The NHS claims that obesity costs billions. If that's the case, the government needs to come up with some effective public health messages, which don't make it sound like a 'nanny state'.