If what you were saying were true, we wouldn't have wildly divergent (macro) diet and nutrition claims being made in cycles without clear resolution. Most of them are wrong, it seems, but the science is hard.
You are right we have some pretty good information on deficiency problems with key things like iodine, B12 (topical) etc. We have much less understanding of how even dietary source actually work even with some key nutrients outside of lab conditions, and beyond that dietary nutrition is absolutely full of handwaving. We are nowhere near a clear picture; lot's of people will tell you we are but they still contradict each other regularly. This is not a mature science.
The hard part of modern diet isn't macro composition, it's measuring caloric requirements and sticking close to them over long periods of time. Macro composition can help with that, but eating way too much of the perfect diet is still going to be bad.
Sure, we understand "too many calories is bad" also, but that wasn't my point. A lot of conflicting claims about macro composition are made for example, and it's really hard to definitely dismiss them (or even answer how much it matters) mainly because we don't understand it well enough.
You are right we have some pretty good information on deficiency problems with key things like iodine, B12 (topical) etc. We have much less understanding of how even dietary source actually work even with some key nutrients outside of lab conditions, and beyond that dietary nutrition is absolutely full of handwaving. We are nowhere near a clear picture; lot's of people will tell you we are but they still contradict each other regularly. This is not a mature science.